I am the Byz at about 1510 and have taken Greece, Anatolia, Armenia/Caucasus, the Levant and Egypt back along with a large swath of southern Russia colonized from the GH. Venice, Serbia, Wallachia and the Ukraine are my allied vassals. I sold the western Russian provinces back to Ukraine to get her to 8 provinces and kept the Eastern ones. I just obtained an alliance CB against Muscovy, she declared on Georgia (which she can't get to mind you as I completely surround Georgia but what the hey). So I am considering a blitz on central Russia but really don't want all those provinces as I will get the over-expansion penalties. Is there a down side to attacking Muscovy and handing over the provinces to the Ukraine, i.e. are there problems with very big vassals? A good 15-20 province Ukraine would do wonders in my eventual war against super-blob HRE Austria.
They will face pretty much the same problems you would face with having loads of poor revolting land except they will not have your powerful human mind to deal with it. Not to mention they lose half of their tax income to you (that comes straight out of their coffers and into your teching money)
They will likely have terrible tech and be piss poor until the land cores. If they have land which spawns lots of rebels or land that they do not have a drect connection to they will almost certainly lose that land to rebels without help.
But if you nurture your massive vassal (who will never betray you unless you break the alliance, which can also happen if one of you switches religion) they can be a very powerful helping hand. I had a 52 province Brittany help me out mssively and they kept growing through colonisation.
All the land I plan on giving him would have direct land connections, be the same religion and I think will be the same or similar cultures so his rebellion issues will be low. I have direct land connection with him so I would help crush any rebellions. We are also the same religion and I have dedicated myself to spreading Orthodoxy across the globe so I'm not changing. Mecca and Rome are my next targets after having converted all the Med coast, Armenia/Caucasus, Anatolia, some of the Balkans and much of southern Russia. Can a vassal form Russia if I get her big enough? A crusade to liberate territory all the way to the Baltic and give it to the Ukraine sounds like fun.
A vassal can take decisions but make sure they actually qualify, Ukraine cant form Russia off the top of my head.
You also need a direct land border to a horde if you want to fight them, a vassal will just instantly give you a truce when the 'nomads are restless' pops up.
If a vassal gets too big you may not be able to annex them without going over your infamy limit.
Oh pooh! No Ukrainian dominated Russia, such is life. I'll still pump her up with provinces as a balance against Austria. I don't need the provinces, I am heading off to India so fast I am always on the brink of over-expansion. Thanks all.