Rehearse voices separately if possible.
Singers need to hear themselves, hear their parts in relation to each other, match vowels, etc. Sometimes what they’ve been practicing while preparing is going to be different from what you need when you get together. It’s extremely helpful to work with them separately from the band. (It’s especially helpful when you need to find the best key to place a song.)
Once you bring in the instrumentalists, the sound techs, and the media tech, you have a lot more puzzle pieces to put together. You do NOT want to be working out vocal parts at the same time. (If you have to, carve out some time when you can dismiss the instrumentalists.) It’s about being efficient.
When you hit with everyone, it should be about putting it together, running it, and working out decisions that affect everyone.