obviously i cant comment on what constitutes an empire as far as the mechanics of vanilla or of the mod are concerned. and i referred more generally to the ck timeframe rather than any specific scenario. as far as i can tell, dali later monarchs retroactively used emperor as the posthumous title, but employed chinese era names. vietnamese monarchs since the 10th century generally conceded themselves as chinese vassal kings in diplomatic dealings with china but referred to themselves as emperor within the state and issued their own era names. chinese style tianming/divine and universal rule was imitated widely in chinese neighboring countries, even if these generally had little desire to claim the world or even china and employed tianxia mainly to described their their own realms--which actually was also the case with even china as time went on. Even the prim and proper korea sometimes had paricularly frisky monarchs issuing their own era names. this is something of a problem in the game when it ties chinese imperial with 'pretender empire.'
You can already have an empire that's mechanically
very similar to being Chinese Imperial without being a pretender, since the differences between Confucian Bureaucracy and Chinese Imperial are fairly small; the latter has the Mandate of Heaven, CBs for (re)conquering the de jure of China and the China region, and 250 times as many concubines (with the resulting extra children being a thing because we cheat behind the scenes as vanilla caps out at/about 9 living children/ruler, discounting bastards, the final birth being twins, or other exceptions we can't touch), plus some behind-the-scenes modifiers to a few event weights that should/shouldn't favour the emperor of China/pretenders, which a non-pretender realm with a similar government would not automatically deserve to be part of.
The Mandate of Heaven is a double-edged sword that's scripted under the assumption that you want to rule China (and changing it to not be about that is not trivial), the CBs for taking China are obviously something tied to ruling/claiming to rule China (and something that others should not be given, much how random realms shouldn't get the Roman Imperial Reconquest CB), and the extra concubines and children are a thing to both make sure that the Emperor of China can accept concubines sent as tribute and to make it more likely that there are princesses for Imperial Marriages, and there's little reason to make any of those things available to everyone that's "Chinese enough" (or Shinto), particularly not when it would complicate things behind the scenes to a large extent as the standing assumption of "Chinese Imperial and emperor-tier" being the same as "China or a pretender empire" would no longer hold true.
I don't really see a good reason to either redo (and complicate) a bunch of things to handle "Chinese Imperial and emperor-tier, but not a pretender or China" or to create another government that'd basically be a slightly fancier Confucian Bureaucracy for emperors.
As for holding an emperor-tier title instead of a king-tier title, it partially comes down to whether it would result in better or worse gameplay and/or AI behaviour. A Dali (or whatever the name was at a given time) that's emperor-tier would stupid itself to death in AI hands because non-nomadic emperors are set to not want to be tributaries of China (and a "unless you're Dali" exception causes more potential problems than it solves) and a Dali that's not a tributary will be gobbled up by China (and Burma, if the new empire was titular rather than de jure) in short order because it simply can't fight those off consistently. A Vietnam that's emperor-tier would cause playability issues for Champa due to de jure claims (and giving them a titular title would be silly) and would also result in the realm having an equal interest in acquiring the de jure land in the south (that it didn't hold much of in the era) and taking back the land China holds that they actually got back during the era if memory serves. Korea would be less likely to stupid itself to death or to cause major issues due to de jure desires (though Jeju would notice it as that's not de jure Silla or Goryeo and those are the only kingdoms around for long (Hubaekje is fighting a losing war in the IC start that'll result in the title's destruction if they lose)), but on the other hand they're reasonably able to be able to get the prestige and realm size (200 holdings, which should be possible with Prosperity) necessary to become a pretender empire without expanding somewhere, and that feels like something that they shouldn't really be doing if they're contained to the Korean peninsula, and there's also the historical submissiveness to consider, plus the fact that you'll devalue Balhae if you make Silla/Goryeo emperor-tier (and making Balhae emperor-tier is just empire inflation).
There's also the size issue. Empires in CK2 tend to be rather large unless they're in decline (e.g. the ERE throughout most of the game) or they're squabbling successor states (e.g. Nikaea and the Latin Empire) fighting over the corpse of the empire that spawned them (which is where the FDaTK emperors will fall), and there's precedent for demoting smaller empires to king-tier (see Trebizond). Dali and Vietnam both hold very little land, and they're neither successor states or declining realms. Korea is reasonably sized, unlike those two.
Mechanics-wise, I don't really see any benefits from having Dali or Vietnam be emperor-tier as I don't really see any mechanics to add to either that'd be easier to handle if they were emperor-tier than if they're king-tier, unlike e.g. Japan where the future need for a Shogun and the need to potentially have a Regent that's got playable vassals under the Tenno while there's a Shogun around means all four playable tiers are needed. Korea would get the benefit of being able to de jure war the whole peninsula (plus some islands), which might be good, though I don't really see anything else being gained mechanics-wise as chances are there wouldn't be a need for king-level vassals without expanding far enough outside the Korean peninsula that you'd already be able to create the empire title during the game.
As for the vague "conquered large areas/various people" definition of "empire", while both Nanzhao (at the time of its founding, which was before the CM start) and Annam (following the Anarchy of the 12 Warlords) conquered certain areas their conquests were fairly modest and feel more similar to e.g. Haraldr Fairhair uniting Norway than e.g. Song reuniting China, and there are other medieval realms that conquered significant areas without being considered empires in CK2 (see e.g. the Seljuks). The unifications of Korea are rather more impressive than those, though whether it's impressive enough on its own is harder to say.
Summarizing the above, it really doesn't seem like you gain anything good for Dali or Vietnam other than "accurate ruler titles" and that you create new problems if you give them an emperor-tier title, so I'd say there's not really a good reason to make either of those two emperor-tier. Korea is more iffy.
How about "King-tier, but called (the culturally appropriate version of) 'Emperor' "? There are two options for setting that up (as there's no "kings_called_emperors" equivalent of "dukes_called_kings"), and neither of them are good. The first is to localize a specific culture to use "Emperor" (or the cultural equivalent) where it currently uses "King", which either would cause confusion due to emperors and kings using the same title or would mean replacing the emperor tier title with something (and it'd also possibly result in petty kings called the cultural version of emperor if the culture has petty kings, which is silly), and which also would result in every king-tier of the culture everywhere using "emperor", which isn't necessarily good (e.g. if we did it for Korean then Balhae would be ruled by an "emperor" and a Korean who became the new petty king of Jeju would be an "emperor"). The other option is to use a custom FOA and custom ruler title for specific king-tier titles, which would break cultural localization (the ruler of the Roman Empire is
always called an Emperor (or Empress), not a Badshah, not a Kaiser, not a Huangdi, etc.) and would result in silly things if the the title is a subject (e.g. Dali (Yunnan) as a subject of the Mongols or Yuan should definitely not be ruled by a local "emperor").
as for the designation '10 kingdoms,' the name seemed to be derived from the longevity and power of the states rather than whether or not the rulers posited themselves as 'imperial pretender.' most of the rulers of these 'kingdoms' and not just of min honored themselves as emperors. if we take all of these guys at their words, we'd have more than 15 empires/dynasties if counting tiny rump states like yin and yan.
I'm fairly sure the only states pretending to be emperors of China on 936.8.7 would be Later Tang, Later Jin, Later Shu, Yue/Southern Han, Wu, and Liao based on the information I've found. All of those other than Liao will count as full pretenders and thus be empire tier, with Liao becoming a pretender if they settle using their special decision (as we can't really have pretender nomadic empires), by virtue of the rulers there claiming to be emperors at the time of the IC start and thus deserving access to mechanics related to uniting China (particularly as the player might want to start as a weaker emperor and attempt to unite China) and the Mandate of Heaven mechanic (the extra concubines don't add much to these realms as they're not the Grace target, but creating a "Chinese Imperial, but fewer concubines!" government for the FDaTK emperors is not worth it).
Jingnan/Nanping never claimed imperial dignity, nor did Wuyue or Chu, based on the information I've found (at the very least, I'm confident they didn't claim all of China at the time of the IC start, which is the
only thing that matters as far as gameplay is concerned), which should cover basically everyone else in the IC start other than Da Yining (I've not found a lot of info about them, but considering neither Nanzhao nor Dali claimed China I'm prepared to call them a non-pretender) and perhaps some tiny realms between Da Yining, Later Shu, and Chu that there's basically no information about (the maps I've seen don't really cover that area in detail), which I'm prepared to declare non-pretenders by virtue of there not being information to the contrary.
Former Shu, Yan, Yin, Later Liang, Later and Northern Han, Southern Tang, Later Zhou, and any others I've not mentioned wouldn't be around in the IC start. We can easily create a few extra titular empire titles to contain the ruler lists for those if we add the rulers in the history files (all the ones I named other than Yan and Yin are already in the history files as of shortly before when Dev Diary 31 was written) and there's not already a FDaTK title to add them to (e.g. Former Shu'll go into the Later Shu file), without affecting gameplay beyond possibly giving someone that's alive in a certain start date a Chinese Imperial or pretender bloodline if they're the descendant from a pretender. We could even conceivably add dynasties in other eras that are irrelevant down the line (as long as they're from after year 1) if we feel like it, though it's really not a priority to do that as we've got many other things to do and dev time is finite.