I know im late but i will put this incase people wants to know what they do.

A perfect Vassal is literally a Copy of your Current Everything really. Not planets or anything but Tech and other bonuses are given to them. Not only that but they are always Loyal to you however want to point out for Federation or Galactic Community stuff they will vote against you if they dont like the idea. They will also say no to a lot of trade stuff if you want something from them like a system. Which is dumb because like... I own you type of deal. Perfect Vassal are 100% more Powerful than any of the AI Empires and if you are on a warpath but dont want to manage anything get a perfect Vassal they will do everything right for the most part and you can even get them to produce a large Armada by giving them Resources via the Edict. For me if i am trading with them i use the Comman Yesmen because honestly i own them they should give me what i want.

Something you should do is capture systems around them then trade them the system. They are a slow starter so giving them a boost is a good idea. You can also pick systems you want to give to your Perfect Vassal before hand which can be found in the Colony Decions. It will give them the entire system. Generally before you do anything you want to pick a spot for them probably next to you and have their empire surround you. You dont need to upgrade the other systems but you can if you want but definetly make their home system stacked so they dont need to wory about anything. I also suggest making the Vassal Your Species as it will copy your civics and traits as well so you have cheated trait points and civics they copy it.

One thing to add is if they arent functioning the wway you want or you want to help your Perfect Vassal and make them even more stronger wait awhile and let them build their empire up then Integrate them to your empire and go to each of the systems they own and prep it to be given to your Perfect Vassal then do what you want with their territory then give it back to them.

I think there should be an option to give them fleets other than giving them resources as they dont have the best ship components usually.

They are now widely extended with the "Regentmaker" features from:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2211461998