The doctor should have given you some nystatin cream, it works faster than anything OTC. But any OTC anti fungal is fine and won't hurt her, Vagisil is perfectly OK. If you don't see some improvements in 48 hours, call the doctor back, get some nystatin and some liquid oral anti fungal too. And don't use babywipes with any alcohol in them, if yours does and you can't find the alcohol free ones, you can make your own with a soft paper towels, water and baby shampoo.
Also when you change her, let her bottom get as dry as possible, even leave the diaper off for 10-15 minutes and let it air out. The yeast grows in damp warm environments.
Both my kids were little yeast creatures, I feel your pain. But pay close attention to what it looks like,(those small red bumps) it looks slightly different than a diaper rash, that way you can jump on it quickly before it gets bad again. Also, any time she is given a antibiotic, while the doc is writing the script, have him/her write you an antifungal too, that way you don't have to go back when it breaks out. Increase her yogurt intake and she should have less outbreaks. Good Luck.