Piano Skills Required

Playing with all fingers without looking at your hands / keyboard; keeping your eyes most of the time on the sheet music and having an idea of where the notes are in relation to your hands and fingers.
Good control of your right hand to play the the vocal line or exercises and learn them. It is good to be a good sight singer, but sight singing may tire the voice (we concentrate too much on the correct notes and in turn pay less attention to proper singing, posture, support, etc. We also feel less secure about the notes which makes it hard to sing properly with the right energy and flow of air).
Playing the bass and some of the chords - to hear and feel the harmony.
Playing a few bars exactly as they are written (in a slow tempo, if needed):
- to get a feeling of the rhythm and texture of the piece.
- to be able to find your entrance after a long pause in singing - learn to pick the note from the accompaniment.
Identifying, forming and playing chords (quickly) from guitar tabs (Am, C, F#m7 etc). Necessary for non-classical, but can sometimes be useful for classical vocalists as well.