It could be a fungal infection in the corners of her mouth. I forgot what it's called but moisture or vaseline will just make it worse because it promotes fungal growth. The best solution I found (for myself) was keeping the area dry and sudo cream.

It could be hand foot and mouth.

When my daughter had it, the blisters started on her vagina (thought she had developed an allergy to her diapers) and then moved to her toes, then her mouth then her fingers.

When my nephew had it, the blisters were around his mouth and inside his throat.

But it could also be a drool rash... just keep checking her to make sure it hasn't spread