Diagnosed with Bacterial Vaginosis. Taking metronidazole 500mg. Is this going to cause a yeast infection and how can I prevent that?

Maybe but not likely. Mixed vaginal infections (caused by bacteria, fungi, viruses, BV...) are not uncommon. If you take metronidazole for a prolonged period of time, there is a risk that the yeasts in your vagina may cause you some transient symptoms which will resolve spontaneously. You may simply want to wait or consult your gynecologist regarding yogurt, Diflucan (fluconazole) or other preventive measures appropriate to you.
Vaginosis . flagyl kills the bad bacteria IN THE VAGINA, allowing good bacteria to flourish. this DOES NOT PREDISPOSE TO YEAST. Taking an antibiotic for an infection in your lungs or sinus... Kills the good bacteria in body including the vagina which predisposes to the yeast .