Personally, I've always thought of them both being the same thing. A musician who happens to sing or vocalise notes.
When talking to other musicians (or non-musicians for that matter), my definition would probably be whatever first came into my mind at that precise time.
For instance, when talking to certain musician friends of mine, we would call a singer/vocalist....'a Warbler' (being very vocal), or even a 'songster'.
Hayley the Classical Vocalist for Hire - Surrey (warble-entertainment.com)