Vassalage is one of things that looks good on paper but in practical terms doesn't pan out as well.
-It's expensive. This directly offsets the "free" unit support, and in large empires it will end up costing you more to run it than anything else other than Bureau or Theocracy
-Which leads to the next, point: If you can run Theo, you don't need it, and you shouldn't, because Theo is expensive itself; using them together can wreck the economy of a large empire
-The value of the experience it gives (+2) is questionable in its window (before Nationhood or Free Speech). At the point at which you might have it, it's quite easy to have a barracks in each city. If you're using mounted units, the stable eliminates the need for it at all, since Vassalage alone won't get you that 3rd promo with Rax+Stable--you need Theo as well.
-For other units, like siege or infantry, they tend to be more expendable or likely to die than mounted anyway (which you want to stick around and keep stacking promos on to rip through more and more territory), so an immediate second promotion isn't all that it's cracked up to be for these guys. It's nice, but when you're throwing out suicide catapults and mopping up with C2 Elephants or CR 1 Maces afterwards, it's not really hard to replace the siege and stack up promos on the clean up crew.
-Cannons are another story. Having CR 2 out the gate is very powerful, enough to have cannons survive to become highly promoted when cracking cities open. But this is at a much later point (Steel). Trebs are kind of a fence sitter, but I tend to find they don't need much help as the massive +100% modifier helps a lot due to the way the combat system rolls.
Nationhood is something I never used to consider (would always just blindly go Bureau until Free Speech later) but have been playing around with recently. Ignoring the drafting aspect for a moment, it seems to offer more than it seems at first:
-Very cheap. The polar opposite of running Bureau or Vassalage in terms of cost. Large empires especially can feel this effect. As a big fan of Organized, this alone makes me like the civic.
-Barracks, as mentioned, are a cheap commodity and are easy to emplace long before you even get the Nationalism. In addition to the point about devaluing the free XP Vassalage gives, they net you a free +2 happy, which is very welcome when whipping up an army and warring/accumulating weariness.
-Those are really the big points, but hey, extra espionage. Great?
And of course, it opens up the draft, which with the proper techs in place can be a powerful method of raising an army fast. The barracks happiness works right along with this, giving you more cushion.
I hardly ever use Vassalage anymore. I expand a lot in most games and it's too expensive to justify. I make a point to start some religion spread before pushing, and I usually have at least one of Christianity/Taoism/Confucianism due to bulbing, so Theo is there for at least some of my cities when build up time comes.. Cuir rush is my most common pursuit so it really comes down to: what is the value of +2 XP that can't even give a promotion by itself vs. less expenses and more happy?