for starters, I think vultures are actually pretty intelligent, definitely not intelligent as maybe a slugcat or scavenger, but maybe similar to a crow's intelligence?
they probably served as like organic waste disposal, being fed the dying,dead, or just no longer useful purposed organisms. a behaviour that led to them being scavengers, like our real-world vultures.
for king vultures it was probably the same, just for more tougher and heavier organisms.
and as for their mask, I think they're simply just secreted from the skin of their face, still able to be removed though ( this would be the worst case scenario for a vulture though, as their mask signals a vultures rank )
also a fun thing to think about is that they like shiny things.. from when their previous diet required for them to break open metal boxes to eat. ik a lot of things are shiny and metallic, but it's still a funny idea