They've changed the logic to require you to be a good deal larger, or at least richer, to diplo-annex people. No more growing from a 2-3PM to a major power by peacefully eating OPMs. As Bavaria, for example, Ansbach would not agree to vassalization until I conquered Salzburg (my theory is that the sooner I do something bad the sooner people can start forgiving me). You could probably derive a formula or approximation from that, Bavaria's pretty small, four provinces, but also pretty rich. Here's the math (I'm hoping that nothing else of consequence changed, but I have no clear recollection, so caveat emptor).
Base taxes:
Ansbach = 5
Bavaria = 38 (12+6+14+6)
Salzburg = 10
So 5/38 = 13% is too big, but 5/48 = 10.4% is not too big. That's my quick take. I'd look around for someone with 10% or less of your base tax, or else force vassalize (is the AE for that now the same as outright conquest? I'd thought it was half before). And yeah, I wanted a longer border with Austria cause I'm a genius, and because border friction really isn't that bad anymore. Seriously it's not been a real issue.