Damn I hate vocalists!
First, I strongly recommend you try to get ahold of Teddy or Moke, who record in these types of situations frequently.
Caveat: I'm no expert, and this is only how I'd approach this. I've never had the specific situation you discuss, but I've run a bunch of onstage when an unexpected vocalist has shown up. I've had surprisingly good results of accidentally capturing the vocals with Healy Method Omni's, and I've had last minute luck by placing a pair of cards close to the vocalists or the vocal monitors. It won't sound spot-on professional, but given that you're not going to have a mixer and you're limited to 4 tracks, here's my suggestion:
Suggestion 1: Split Omni's on stage, behind or on the same "line" as the vocalists. DIN cards on stage, about 4' in front of the two vocalists (try to get the vocalists to space about 5 ' apart).
Suggestion 2: On stage Healy Method Omni's 3' in front of the 2 vocalists (try to get the vox'ists to space about 3' apart), then at stage lip, run DIN or ORTF cards (config depends on how wide the spread of musicians are, so most likely DIN.
Suggestion 3: Stage lip Split Omni's tapered to a single channel. Stage lip center DIN (or ORTF) cards. Another card directly capturing the vocal monitor. If there's a separate monitor for each vocalist, then 2 cards tapered down to a single channel.
Approaches 1 & 2 allow you to not have to "directly" mic the vocalists, saving a channel for you. Additionally, I doubt that the monitors will do much harm, and will probably help with the vocal capture, over all. The risk, of course, is an echoic effect, but it's probably a low risk.
Good luck, Bay Taper!!! I bet you'll do great with this, whichever course you eventually settle on!!!