Does anyone know the difference between vulval melanosis and vulval melanoma...

Melanosis is the benign pigmentation of the mucosal surface of the vulva.
The areas are multiple, flat, and stable. Biopsy, if performed, will show clusters of melanocytes with a benign appearance.
The cause is unknown but genetics presumably plays a role. Left alone, they will remain stable for long periods of time, but may fade following childbirth.
As they do not pose a threat and are not a cosmetic issue, no treatment is necessary.
Suspicious areas or areas that are rapidly changing should be biopsied.

best to check out the following site for explanation on vulval melanoma.
http://www.dermnetnz.org/site-age-specific/vulvar-cancer.html