In EU3 once someone is your vassal they become a slave of sorts. Generally speaking they can never get out from under your thumb as they pay tribute to you and you were already big enough to take them down. There are exceptions of course with Sweden and Brandenburg usually revolting but generally speaking once they are a vassal they stay that way.
What I like about vassals in CK2 is 2 things. #1, you have to keep them some what happy. You bring them into too many wars they they will revolt. #2, they are more useful. Vassal will actually try to take land that they feel is their land which can also lead to #1 if they get too big. There is often times where a vassal of yours has a claim on some province but you can't easily integrate it into your realm because you as the superior in the relationship don't have a claim.
Another area that is interesting in CK2 is being able to raise troops from the vassals. I'm not sure how that model would work with EU's gameplay model. But, it would be nice if you had a little more control over where your vassals(and allies for that matter) attacked. Often times a bunch of smaller allied nations will have 2-3k troops and wont attack a siege of 7-8k even though the group is 10k or more. Perhaps more goal oriented wars like CK2 has for land would help with that. For example, why do you need to occupy half of the Mamluks when all you're really after is Alexandria? I think CK2 does that really well and hope to see a similar system in EU4.