Depending on set up time i would take line out from the P.A console and setup to ambient mics on stage.
Ultimately you could use your mixer with as many as 8 mics but its more of a risk.(usally if your rushed things dont work out as well).
The advantage of a signal from the P.A console is that the vocal will be clear, but depending on the room you will get a wash of all the other instruments underneath.
Usally in a bigger room the mix will contain more signal(you dont need to mic cymbals in a small room) because the stage spill wont travel.