I have one 2.2 game under my belt, and I did take a lot of vassals - but only after I was in the lead and just wanted to subdue opponents and move on to the next. Couldn't say whether the lack of help I noticed was due to the vassal system being weak, or the empires I was subjugating being left in my technological dust.
I found subjugation wars to be a way of putting large enemies away without having to do multiple rounds of claim and conquest, and also without adding more planets to manage. I did not create any vassals out of my own territory.
I found subjugation wars to be a way of putting large enemies away without having to do multiple rounds of claim and conquest, and also without adding more planets to manage. I did not create any vassals out of my own territory.