There is no written rule as to 'what should my volume be', at the end of the day, abide by the the rule of thumb for pretty much all recordings - mix until everything in your track is balanced, and make sure nothing is clipping!
As for mastering, no single process is ever correct, it all depends on what the style and final effect is like, but regardless of whether you have vocals, or violin, or kazoo in your song, you are still going to be loosely looking at the same process, you just may select a different compressor, or treat the audio in a different way. Just think of the voice as another instrument or element of your beat.