Hi,
I a'm about to play with vocalist in duet situation and since I haven't been doing this
I am wonder if you guys have any tips how to approach a song when I don't play main melody
but just accompaniment. So far I've been playing mostly solo or in a band where I did more comping then accompanying
which requires I guess fuller musical palette. 

Yeah use your left hand to play a bass line and your right hand to play chords. The bass line will add rhythmic interest and add depth to the chords.

Keep the chords voicing around middle C as much as you can. Choose inversions to do so.

Don't play the melody!