I was an instrumentalist long before I ever started singing (in public). At our local folk club I would sometimes play an instrumental for my turn and the next performer might say, "I'd like Jim to stay up here to help me out with my song." In that case, I didn't feel guilty about taking an extra turn.
There are some fiddle tunes that have a verse or two, but for the most part are instrumental tunes. In this case, it would be considered the instrumentalist's turn, not the vocalist.