Be sure to see O if you get the chance, it's quite a show. We had our National sales meeting this last March (in Red Rock), and the crew paid a visit to Bellagio on a night off.
Fortunately the waterjets will be lit so you will have some back lighting to achieve a natural exposure.
As a fail safe, your camera should have a night portrait mode option on the dial (read the manual). If you become lost in the settings, you can ditch and use that before throwing the camera. Be warned, it might choose too slow a shutter, even at iso 400. The reason for that is although the flash will instantly light your subject, which is the couple, the shutter needs to stay open a little longer to allow the much lower ambient light in the backdrop to expose. The lower the light, the longer the exposure. Essentially your are taking two different shots on the same frame. In which case the couple has to hold their pose a bit and not move.
You could set you camera to flash on second curtain (if you have that feature and that it works with external flashes if you use one). That way the flash will go off just before the shutter closes. People tend to flinch or close their eyes upon seeing a flash, and such actions will show up during final moments of exposure as blurs on them. Come to think of it, you'll need a tripod. Maybe someone or the even the hotel desk will have one you could borrow? Otherwise, you'll have to crank up the iso to get within hand holding range and risk noise.
I suggest that you do some night time testing in your home town before heading over. Congrats to the lucky couple, and have a good time.