Is vision a vestigial sense for deep ocean species?
I can't answer your third, but I can answer your first two. With one word, in fact:
Bioluminescence
http://brightnepenthe.blogspot.com/2010/08/palate-cleanser-90.html

That's the deep ocean at night for ya. Unlike underground environments and caves, it's not pitch black pretty much anywhere in the ocean. There are things to see everywhere, and they play an important role in predator/prey relationships and/or mating. The absence of natural light does make things harder to see, but that's why you see the enormous eyes on species which live far down. So, nope! The eyes on deep ocean critters are not vestigial. They're very functional (and, if anything, probably increased in sensitivity).
The basics are summed up here: http://www.lifesci.ucsb.edu/~biolum/functions.html