18 month old girl with pimple-looking spots on vagina. Not responding to diaper cream, what is it?

Could be a yeast inf. Depends upon what diaper creams are being used. Any otc creams will not contain an antifungal medication that may be necessary to treat the rash. Please be sure to have your pediatrician examine your child to confirm that this is what the rash actually is so that proper treatment can be prescribed.
Get it checked. Could be a staph infection, candida, or other dermatitis. Please take child to your doctor.