Vassalage + OR does tend to work best in expansion wars, but the more I think back on the games where I used Vassalge to advantage, the more I notice that I was mainly using it in conjunction with several other conditions for 4th level units out the gate.
That would mean 10 XP.
Here's a probably plan where I would want to shift into Vassalage:
I've already got 2 GGs and settled them, or Charismatic and 1 GG (not at all hard with Persia, btw).
Assuming you're Persia, if you get 2 GGs early on, you can settle them in two separate cities, then switch to Vassalage + Theocracy and Barracks to get 9 XP for CR3 Swordsmen or Trebs or Cannon in two key production sites for your elite corps. The remainder of your cities will not need Barracks, even, because Vassalage+Theo already gets them to 4 XP for level 3, so you can save up on infrastructure in peripheral cities that way until you max out with unit production in every city once you switch Civics. Assuming you're coming from Barbarism, the Vassalage change will cost minimal extra and will offset the costs of extra troops, especially as you gain more cities.
This is the baseline "good position" wherein I can see a good swing point for Vassalage (don't need to build Barracks everywhere, massive troop XP advantage) that I would plan for the switch at the opportune time. Best, of course, with early SE focus for greater point troop production at the switch point.
Spiritual Civs can make use of Vassalage's XP advantage if they don't have a Commerce-heavy capital and have no Wonders they want to chase down. Basically, you trade a 50% hammer advantage in your capital for +25% hammer advantage in your peripheral cities while still maintaining moderate troop production on the XP Civ (for garrison or to prepare for the next war). Good for post-war development.
There's also some use for it if you have a hammer-rich secondary city that makes units and Wonders. Magic number is 10 XP, of course, so you'll want Vassalage, Theocracy, Barracks, and 2 GGs. If you're Imperialistic and Aggressive, you can try to go for 4 GGs in two production cities, effectively giving you the Persia setup above, except you'll have Barracks everywhere because you're Aggressive.
Of course, Imperialistic and Aggressive is Genghis Khan, and that also gives you access to the Ger, which is another good reason to aim for extra XP. Basically, Ger+Barracks gives you 7 XP base for mounted units. With Theo+Vassalage, you can come out with Combat 2 Shock Keshiks right out the gate, or go for Combat 3 Keshik with an aim for Blitz on the field. HAs are more powerful now with the removal of the -10% City Attack, so this is actually worthwhile to acquire. Combat2 Shock Keshiks are godly, able to take out even mildly promoted Spearmen - their counter unit. Send out Spies to knock out city defenses and your Elite Keshik Stack can take out several cities very quickly.
Of course, since you're using Ger and not GGs to push your Keshik, you can use your GGs on nonGer-cities to produce CR3 Trebs or Swordsmen or Formation Axemen or whatever suits your fancy. Or, make a SuperMedic AND make a SuperKeshik with Blitz, Morale, and added withdrawal chances.
Basically, Vassalage is useful for those situations where you want to push your unit XP past that afforded by Theo, or for some reason have no use for Bureau.