It's almost impossible to get the subjugation casus belli as is - you no longer get it by demanding vassalization and having them refuse - and now only get it when the subject empire is "Inferior" in Military, Technology, and Economy. Which is pretty hard to do, especially early game, and by the time you are able to do that, the vassal is in such a worse-off state than you that it's almost not worth it to vassalize them. If you can't force them into a specialization by force, that's fine... except the minimum relative requirements to make them a specialist later by altering the contract (if you even can, haven't gotten to this point yet) are higher than is what is required to gain the subjugation casus belli. This means that the entire concept of specialist vassals are not available if you are going the conquest route... which seems directly counter-intuitive, as you need to be excellent at everything in order to forcefully get a specialist who's entire purpose is to focus on one thing so you can focus on another thing that you don't need because you had to focus on it just to get them in the first place? Huh?
It also means that you aren't likely to force vassalage on another empire until later in the game, at which point you probably shouldn't need them given the requirements being so high anyway, and again, you can't make specialists by force... which again isn't relevant because by the time you get the casus belli to subjugate them, it's no longer useful to do so.
Not to mention as others have pointed out that vassals have way too much say in refusing contract changes, that should be reflected by serious penalties to loyalty and attempted rebellion, rather than them just saying "no" when by rights they should not have that power. (Though I can see why that's the case in regards to dealing with player vassals, as a goal with this system was to make playing as a vassal more interesting, so that's a concession I can see being made for that reason.)
Only overwhelmingly military force should be needed to force an empire to become a vassal. Why can I occupy their entire empire, blow away any ships they make, and bombard all their planets, but haven't figured out how to demand their subservience? Why do I also have to have a massive technology and economic lead too? Will they not bend the knee to someone who doesn't have at least 5 models of iphone ahead of them?
In addition, refusing a vassal contract should grant you casus belli to enforce it, as it did before. Refusing an ultimatum is often a casus belli in the real world. (Justified or no.) Stellaris should be no different, and why this was taken out I don't understand. Make it so you can't demand vassalage unless you have overwhelming military power, but technology and economic power should not matter. The one with the bigger stick more powerful fleet should have the say here.
(The negotiations have failed, and force is now necessary. That I intended the negotiations to fail is of no consequence.)
The current system is designed entirely around making vassals peacefully, and heavily prevents militarily acquired vassals, which would make sense for an expansion titled Federations, but not for one titled Overlord.