I think you've answered your own question, if you put a bass line down using a bass then your a bassist, if you do it using your voice, vocalising it, then you're a vocalist surely?
I know Flea uses a black Stingray 5 on some tracks, Zender had a pink Stingray five, and Artis has fancy custom basses in both four and five string versions.
Many pit band bassists are pro and most of those appear to be five string players ime, some mighty fine musicians out there from musical theatre groups right upto Broadway!
There are many pro players that use a five string in the studio but not live (or not often at least).
Flea - Red hot chili peppers
Stuart zener - Jamiroquai
Jamareo Artis - Bruno Mars
Off the top of my head.