I dunno, personally I can administer their cities better then they can to meet my particular needs.... so why would I give them back? Unless the conquests are super early, but I tend to be more of a medieval conqueror it seems. By that time, my econ is approaching full swing.
Unless my economy is completely crashing, which has never really been the case... I have been able to maintain 5 vassals on a normal map with the economic mosquito that is Alexander, so I can't imagine it getting that bad if you play it well. Beeline Communism, cottage and windmill the hell out of everything.
This is only on Monarch so maybe it gets significantly worse at higher levels?
I typically find out what techs the un-vassaled AI have and give my vassals at least that much and also gift them the techs as I sell/trade them to other un-vassaled civs. I also typically gift surplus resources I can't sell. Even if they still end up no military or econ asset, it gets me their vote if I want it.
But then my reason for acquiring vassals is mainly to save time by not needing a full conquest to make them irrelevant. On normal maps, I typically want their good land anyway. And if someone attacks them, it saves me the negative diplo for declaring war on someone's friend. I am trying to recall the last time someone declared on my vassal, though.... can't recall.