How to release individual systems as vassals?

[Das Boot]

I remember in the past I could release individual systems as vassals so I could fine-tune exactly how large or small a vassal state was. Recently when i tried it I could only release vassals by sector, and ended up accidently giving away a Starfort I wanted to keep.

Is it still possible to release single systems as vassals, or does it have to be on a sector scale now?

[IPWIW]

sector
you can cut off by removing bases but that's it

[Khorne Flakes]

Be aware that your vassals don't contribute to your galactic community weight, nor will they vote for your resolutions if they don't like them(like any other empire).

[some moron]

Then what would be the reason to do this? Is it just because you can't handle that many systems, or that they are spiraling out of control?
Seriously, if they aren't loyal to you, as the term "vassal" suggests, then what's the benefit ie is there any difference between them and any other competitor?

[Elitewrecker PT]

Depending on type vassals give you some stuff in exchange for less microing. Granted, the real estate is always more useful either way.