I don't think it really matters: the piano or violin just offer a reference notes the vocalist needs to be able to match. For example, even though the piano is a limited instrument (frequency wise) it's perfectly possible for a violin player to practice "against" a piano. In the end, whatever the violinist plays is never going to be perfect. There are basically 3strategies to cope with that: micro-adjustments (correcting), add vibrato (masking) and stop playing that note and move to another note.

I mean in some sense yes, but more importantly, you probably want to practice with a harmony instrument, i.e. one that can play full chords, which probably means piano, guitar... I can't think of any such instruments where it's easy to tune on the fly as with a violin.