I've only run onstage for bands without vocals. However, my rule of thumb (only have cards right now) is to start NOS and narrow/adjust otherwise from there. Vocals on the onstage tapes I've heard are not at all the strongest point, at all. If you're cool enough with the band to run that close, you might want to try taking a feed from the board as well, just to get some vocal presence.
Best of luck. It's a learning experience. If this tape is important, get there early and tweak during soundcheck.