Mmmm dig vibes. I totally agree with Page.
Get a direct patch of the vocals if possible. Put a stereo pair on-stage to catch the direct sound.
If it were me, and without seeing the setup, I'd put two omnis up, spaced say a meter apart and about 2 meters away. If you can fly them great.. put them over her head, sort of like drum overheads for the entire band. If the omnis scare you, go ORTF. Use the extra channel to grab mostly the bass, but not superclose, get some snare too, vibe spill into it is also good. Put that extra mic low on-stage like a meter away from the bass. If there was another channel I'd mic the piano, but as long as the bass and drums are clear the piano is OK being slightly diffuse and distant. The diffuse distance if the piano then sounds natrual and becomes layered 'depth'. Diffused distant drums and bass is mush.
Don't fear the omnis or the 3' spacing (I'm being conservative, even more could be fine) because when you mix it, both the direct vocal (which may need verb unless the board feed is wet enough) and the extra bass/drum channel will firm up and fill out the center with clean clear direct sound and the omnis will bloom nice and wide with ambience clean instumentals and fat sounding vibes. In fact, depending on the musican placement, if it is really piano on one side, singer middle and drums, bass, vibes on the other, I might even orient the axis of the spaced omnis perpendicular to the front of the stage, bisecting the piano and other instruments and superimposing the two sides in the playback image, instead of the traditional way. Can't tell without seeing it though.
oh other posts have happened.. posting anyway..