Hi thanks for asking question. Here are few possibilities of vaginal blister. First blister can be because of sexually transmitted disease like human papilloma virus and herpes. Violent sexual intercourse can lead to blood filled blister formation. The friction on that area might be because of tight clothing can lead to such blister. Blister can be simply because of bacterial infection like that from staphylococci and associated fever might present. Sometime very less possibilities but it can be vascular angioma. So your blister should regularly monitored for few days and trace of its cause has to be search by taking sexual history and physical examination. According to cause treatment has to be done. If it is because from trauma,infection boil then only it will go away without treatment. I hope my suggestion will help you