The only realm that was ever in a vassal relationship to another SA realm was Aquilegia. And I believe that was only after a rebellion lead by carpet-bagger nobles. (I'm not 100% clear on that, I was already gone from Morek when it happened.) Other than that, I don't think we've ever tried to enthrall any realm.
Aquilegia was founded by Morek colonists. Rebels eventually took power, Xinhai didn't like that. There was a war, and it was eventually settled for vassal relationship instead of complete destruction.
This didn't work out too well.
