Are Vassals Good Stellaris?

Vassals are simply not worth consideration on higher difficulties. Vassals will go to war when you do and can't expand to new space. Some people complain that the AI fleet loses drive up War Exhaustion, but I see it as them "running interference" while I take out starbases and planets or my war goal.

Are vassals useful Stellaris?

You can use vassals as buffer states between your own Empire and enemies, so that when the enemy declares war your vassals get ravaged instead of you. These factors alone can make vassals worthwhile.

Are tributaries worth it Stellaris?

IMO tributaries are worth it in the early game. If you're going aggressive. Lots of targets around, claim a few systems, make them a tributary. Claims stuff on a different empire, make them a tributary.