A lucky peace treaty

[Wizzington]

It's not really about luck, AI countries will accept vassalization at the drop of a hat.

You can vassalize even vastly larger countries with superior militaries as long as you have almost any warscore that counts as a "victory", just wait for their war capacity to go reasonably low.

I once vassalized England as a 2-province Leinster with about 20% warscore. It's really an exploit.

 

[Wizzington]

If you have enough self control to do so, just don't vassalize countries unless you have won a 100% victory or it would be reasonable for them to accept (say, if you destroyed their entire army and hold most of their territories).