I think of vocalist as the catch-all term for someone who uses their voice as part of a musical performance. Not all vocalists are singers, but all singers are vocalists
It's not tobacco smoke, I have a strat copy here that has lived in a non-smoking teenagers bedroom for a long time and that has loads of yellowing. I think it's more UV damage than anything else.
I think some setups are more forgiving than others. When you take adjustment to the edge of what's possible it doesn't take much to push them over that edge.
Obviously instrument quality has a part to play, my Yamaha 5er is very consistent