Playing as Byzantium, I have finally won my last war against the Ottomans and I can subdue their nation into a single province. I also have the option to make them my vassal. Having them as a subject can facilitate further conquests of Syria by feeding them conquered provinces and then integrating them later. The problem is that as a historical rival their liberty desire is 50 % higher than normal at all times and our relations are always 25 points lower.
Is it in my interest to vassalise them?