While the vanguard is not worse compared to paragon in terms of equipment. Their role is indeed spot on, as its name suggest, vanguard. Usually forces in vanguard or skirmisher (these two terms in essence have the same role), have a role of somewhat like a scout or heavy scout or expeditionary, these are range of the scout's equipment in regards to how better or fancier their given equipment is, the latter is the better. The vanguard is somewhat in the expeditionary range, as described in the game's lores. But still, they are still the first to advance in the star union military hierarchy, somewhat occupying as an acceptable loses if they die type of forces, compared to the paragons. Other imperial units, i mean units that have imperial tag in front of their names, like imperial behemoth tank have more in common with the paragon than the vanguard, the paragon is also regarded as the official imperial nobility or aristocracy (as described in game's lore).
Usually, sure, but there is an important distinction in that in historical terms a vanguard is usually only a few hours ahead of the main force, while in the case of the Vanguard, they're the only ones on the planet for an extended period. Strategically, they might be a scouting force, but tactically, they can't be equipped on the basis that they'll fall back to the main force on encountering any serious opposition. They ARE the main - in fact, the only - force on the planet until the Void Gate is set up. Regardless of the name, they're going to be set up as a conventional army - the distinction being that they're a conventional army that's operating light years away from home.
And that, I think, is the primary distinction with the Vanguard. They're what gets used for duties which can involve being isolated for long periods of time. It's worth keeping in mind that there is at least one instance in the campaign of a Vanguard force being used as a garrison force on a world that the people who sent them to it
never wanted anything to come back from.
The imprtant thing to remember is that the entire star union military is deliberatly held back. Remember the fluff text on the vanguard trooper: a human military is used instead of robot forces because it is more palatable to the wider population, and encourages them to sympathise with the military, not because it is more effective. It is telling that the last time the union faced an existential threat, the heritors, they invested heavily in combat robots, with a wide mix of exotic technologies standard. Mothballing the RPRs was a decision putting domestic politics over pure military effectivness. If the star union faced another existential threat, I have no doubt that a new generation of combat robots would be the first response of C.O.R.E.
That's... kinda essentially what the Autonoms became in some campaign missions, although there are also Autonoms that are genuinely independent. With respect to the Reapers... I don't think they actually represent a step above what was otherwise available, but are instead an example of something that was specifically made to counter essence technology. Outside of that context, though, are the Reapers really that much more advanced than the PUGs and drones of the Vanguard.
In that context, the fact that the vanguard have gear that is a cut below paragons looks less like a rational military decision and more like a political one. There are no threats to the star union, save for rebellion and civil war. Preventing this is the primary concern of the military. There is no reason to give hard to control frontier troops equipment any better than the cheapest gear that won't be overrun by the wildlife. Even the secret tech mechanics work with this interpretation. We see union era vanguard forces using one of synthesis, voidtech, or promethean gear. Why aren't all of these standard, or incorperated into the main vanguard lineup? Because they don't need to be, and it might make the vanguard a threat. Even the paragons don't have access to this equipment because they too need to prevented from growing too powerful.
Here's the thing, though - you're assuming that the Star Union was assuming that the threats that the Vanguard would be facing would be mere wildlife. I don't think it was that complacent. It's pretty clear from the fluff that the Vanguard is expected to fight wars from time to time, and probably
did in the initial subjugation of the Kir'ko.
With regard to secret tech gear - a lot of it is stuff that was being developed relatively recently when the Cataclysm hit, so it hasn't been fully integrated. And some of it is stuff that is specialised for dealing with specific threats. I'd also note that the reason why you normally see Vanguard using those techs is that the alternatives are the psychic techs and Xenoplague, and they usually didn't get dropped into the campaign without there being a reason
why that commander was using that particular tech (a principle that applied to all races when it came to those techs, not just Vanguard).
Ultimately, though, the reason why it hasn't all been integrated fully is gameplay. Yeah, it's a copout, but in principle there's no reason why the army of the Elven Court in AoW3 couldn't be using every class except dreadnought, and an argument could be made even for that. In practice, this gets reflected by different leaders having different classes. Ultimately, what's probably
really happening when a commander chooses a secret tech is that at some point they decided to specialise in a particular one, when previously they might have specialised in something different.
