This is a good guideline to have a quick read through as it is a good summary of vulval skin disorders. There is a really useful patient hand out on vulval skin care on page 17.
Ask the obvious questions and also about sexual problems, cervical cytology history, immune deficiency, any pads or tampons or condoms used, any family history of autoimmune conditions, any atopy and incontinence (which aggravates or causes it).
Consider screening for TFTs, diabetes and STDs and also ferritin in dermatitis (20% may have a deficiency and correction can alleviate symptoms).