As far as I'm aware melanoma is most definitely related to sun exposure and there is no consensus opinion on vitamin-D and cancer prevention, with the most evidence being for it being protective against certain bowel cancers..
Edit: Here's a source for my view: http://www.nhs.uk/livewell/summerhealth/documents/concensus_... [3rd and 4th bullet points for example]
What 1 MED is for you depends on where you live, what time of the year it is, and what skintype you have. If you take a sunbath of one hour in New York in July, and the next day your skin if mildly rose but not burnt, than 1 MED = 1 hour of sun for you, under those circumstances. Then you can divide that into two or three parts spread out over the week. So for 1 hour, you can sit in the sun for 20 minutes three times a week. That will give you enough vitamin D without causing nonmelanoma skin cancer.
In Mexico the sun is brighter, so that would shorten your 1 MED period. In november in NY the sun is less bright, so that would make it longer and probably even harmless.