While I'm not sure what you're using this for, this might have been one case where vectorizing wasn't needed. If you know what issue to look in or find Marvel promo posters you probably could've gotten a high-enough resolution scan to print decent.
Vector art is a great thing, but sometimes not needed. I'm still torn about reproducing hand-drawn art. In many cases we're preserving old artwork that's hard to come by. On the other hand, many times some of the character is lost of the original drawing. In an interview comic superstar Mark Silvestri once made a comment that the stuff you regularly screw up is what becomes your art style. It can be done well (see Felsir's posts), but it is tricky.
Gradient Mesh tool is definitely the way to go to do that type of shading if you want it to look just like the original. Like Mahuti said though, it can be very tricky and very time consuming.