I have vaginal malignant melanoma, and scheduled to do surgery next Tuesday. However, my doctor said the possibility to be cured is very low.

Gyn oncologist. I would recommend that if the surgeon scheduled to perform your surgery is not a gynecologic oncologist that you see one before your surgery. These doctors have the most training and experience in evaluating and treating cancers of the female organs and anatomy. Living in New York, you have access to some of the top specialists in the world.
Mucosal melanoma. From your question, it sounds like your primary is in the vagina. This is many times referred to as a "mucosal melanoma" (as opposed to the much more common cutaneous melanomas arising from the skin). These tend to be quite aggressive. However, please consult with your physician about your prognosis. He/she has much more information about size, stage and spread of your disease to give you answers.