You can clean the area of the vulva (that is the external area the vagina is internal) off with betadine iodine solution which is safe to use and will help with eliminating fungal or bacterial infections. You can use Neosporin or Triple antibiotic ointment on the sore spot. If the dog licks that it won't hurt her but sometimes using a bit of peanut butter on the roof of the mouth will keep the tongue busy so the ointment soaks in where its needed.
This may be due to an infection from a urine 'drip' leaving the area damp, it could be due to allergies, or it might be a sign of canine herpes virus which you can read about here
I'd plan a vet check up for her just in case its a mild leak of urine causing the irritation or a small infection. She may need meds to help with mild incontinence or to clear up an infection.
To help reduce pain and inflammation you can use a cold damp compress on the area for a few minutes before cleaning and treating.
Hope this helps you!