KRL - from your latest description, it sounds to me like you're simply working in a challenging environment: one with broad dynamic range (big difference between the quitest parts and the loudest parts), including a mix in which the vocals are noticeably louder than the rest of the music. Given those challenges, there's really nothing you can do with your levels (or the rest of your recording gear) while recording to fix the problem*. Your goal with the levels during recording should be to have the loudest peak of the recording approach, but not hit, 0 dB. You'll have to tackle the dynamics and/or bad mix in post.
*Well, there is one option, but it's not related to your gear directly: try recording from different locations within the venue to see if the mix is better in one spot than another. The mix should sound better in some locations vs. others, even if it's a crappy mix. I'd start by trying to set up near the sound engineer because SEs are using their ears from their own location to determine the mix. (A good SE will walk around the venue, of course.) Then, wander around with that as your reference point, moving forward / back and left / right until you find a spot that sounds better than the others. At some point, you'll likely find a spot that sounds markedly better than all the others, where it sounds like you're "in" the music, where it's more difficult to pinpoint the precise source: rather than listening to music blasting at you from that specific stack over there, and the other one on the other side of the stage, it sounds like the music is floating all around you (this is difficult to explain!). This is the "sweet spot". (Finding this spot won't necessarily help a bad mix, but it will ensure you capture the best recording you can given the variables, and will give you a better recording with which to work in post-production if you want or need to edit.)
One of the challenges with the type of field recording we do is that we don't have the luxury of a soundcheck, so we have to do everything on the fly, including select our location (i.e. find the sweet spot). So try different spots during a band about which you don't particularly care, or a different spot each time you're in the venue, to try to get a feel for what parts of the room sound better than others. The unfortunately reality is that the "sweet spot" of the room will shift from night to night, even band to band in the same night. But there are definitely areas of the room that will sound consistently better than others, and you'll find them, even if it may take some trial & error time.