Fun one! Great advice so far.
If you can record three, four, five or six separate channels instead of just two, here are a few suggestions of the top of my head-
Start with an ORTF pair of cardioids, or a not-overly-wide spaced pair of omnis (say 2-3' max) at the front edge of the pit, facing down into it (basically directly behind the typical conductor's position in the pit). They probably don't need to be elevated very high above the pit edge.
Get a feed from the board with the vocals if you can. Recording one channel of that is enough, the vocals won't be mixed in stereo (but sound effects sent through the PA might be, and if so recording that in stereo is better). That's 3 channels.
If you can't get the board feed, but can record 3 or 4 channels, use the omnis at the front edge of the pit/stage, but very widely spaced, about the same width as the PA speakers and on-axis and pointed directly at them, so they'll get clear vocals from the PA. From this position of the omnis will work well for the PA vocals and for on-stage sound, and the wide omnis will likely mix well nicely with the centrally placed ORTF cardioid pair (4 channels), or a single center cardioid (3 channels). In addition to mic'ing the PA for vocals (and effects?), the omnis in that position should provide better orchestra coverage, an increased sense of width and heft to the orchestra, and some nice room ambiance.
If you have 4 recording channels and can get the board feed, you might record one channel of that, the ORTF pair, and use the 4th channel for a single omni, placed either out in the seats a row or two back for good room ambiance and orchestra bottom end heft. Alternately you could place the single omni on stage in the center at the back edge of the pit, to record more of the on-stage action sound. In that position the omni can be mounted very low to the stage, or even directly on it (cushioned with foam padding or a towel or something).
If you can record 5 or 6 channels, and can get the vocal (& effects) board feed, I'd move the omnis inwards somewhat from the super-wide spacing mentioned above, maybe halfway towards the center instead of at the far edges of the pit, and point them down into it. Keep them relatively widely spaced though, at like a minimum of 5 or 6 feet from the central ORTF pair, basically at the 1/4 and 3/4 points across the front of the pit with the ORTF pair in the center.
These option will give you a lot of control over the resulting mix afterwards.
Use your ears to find the best location/caps/pattern during rehearsal. Put your mics on a stand and monitor the sound with some good noise-isolating headphones.