There are several potential causes of your “water blisters” in those areas. Whenever we see skin lesions in the mouth and vaginal area, Behcet’s disease needs to be considered. Behcet’s is a clinical diagnosis as there are NO specific blood tests for Behcet’s but often the Sed rate (ESR) and C-Reactive Protein (CRP) are elevated.
The involvement of other organs (eyes, joints, etc.) is a good hint that Behcet’s may be the culprit.
Lupus can also cause lesions in those areas and yes, in that case, a (+) DS DNA and other rheumatologic tests will help guide you to that diagnosis.
Pemphigus, Bullous Pemphigoid and other autoimmune blistering diseases require skin biopsy for diagnosis which can easily be performed in the dermatologist's office.