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.
There have been realms that have been hemmed in by SA-following realms, such as RE and LE, and even Averoth. But none of those were held in vassalage to SA. They were mostly ignored until they decided to grab the pokey sticks.
