Administrative efficiency and other modifiers should affect convert tributary to vassal

[TheMeInTeam]

The key is the order of operations. You can do the tributary war at any time starting in 1444, way before you get the required admin efficiency. You can simply wait to do the conversion to vassalage until you've got enough Admin Efficiency.

In post-imperialism timeframe (where this much admin efficiency is possible), you can get as many giant vassals as you want in very short order. The rate of vassal/annexation for conquest speed is not gated on the rate at which you ACQUIRE vassals. You can turn the entire world into vassals in < 50 years. The stops are # of diplomats, annexation cost/speed, # of relations (which can eat into points needed for annexation), and opinion (which ties up diplomats to improve).

If you are trying to one-tag with heavy support from dip annex, all of the above are more significant barriers to getting cores through DIP points than making vassals in the first place. To such a degree that giving the land to vassals in first place is a joke. You can even annex arbitrarily large amounts of land, give it all to one client state, and blow up the resulting rebels if you want, generating 500+ development subjects out of thin air on the spot.

Or again, if your modifiers are really that good that you have 75% admin efficiency...just full annex and release Spain/GB/France on the spot, lol. Age of rev abilities will keep giant subjects like that in check with a little investment.

Tributaries are just too easy to get to allow this. You will always be free to convert them to vassals later, even if they grow in the meantime to some arbitrary size. For example you diplo-tribute someone early on, then they grow quite a bit, but you're still free to convert them to a vassal without any hassle later on in the game. And this needn't be some 750 dev regional power, it could be any number of countries you simply don't want to bother with ever going to war with.

From a "balance" perspective, this simply isn't significant to late game WC play. You already paid a larger cost just being a celestial government than you're getting from doing this in return.