How impactful is this in the long run? What happens if your vassal is the opposite alignment?
Good and Evil Alignment
Good Alignment is solid if you want to make free cities become vassals sooner, and will generally have most AI of neutral or good alignments more willing to ally with you off the bat.
Downsides are that you can’t just declare war on people willy-nilly without incurring some penalties and you lose out on xp from world spawns from time to time.
Evil Alignment lets you be more aggressive and expand quickly, at the expense of being friendless and eventually people will declare war on you out of principal.
If you’re going for a military win then it’s the go to for it, and you can still do expansion as well, as cities you don’t burn to the ground can be vassalized and will give you resources and provinces that count towards the total needed. It’s also a lot easier to get pure evil, then it is to get pure good.