Does cephalexin 500mg treat vaginal bacterial infection? Im allergic to Flagyl?

It likely won't affect the vaginal infection, instead try and ask for the Metrogel, which is a gel for vaginal use( you should be fine unless you had a bad bad reaction, the pills make most people sick on their stomach) or for cleocin cream also for vaginal use. You may get a yeast after either of these as they are so powerful they kill the good bacteria that fights yeast. Ask for a diflucan tablet or two to help with that if you are prone to yeast.