xmap creates a mapping for just Visual mode whereas vmap creates one for both Visual mode and Select mode.

As far as I understand, the intent of Select mode was to make Vim behave just like every other non-modal editor when text is selected, i.e., typing anything immediately replaces the selection with the typed text.

It seems to me that you would almost NEVER want to map anything in Select mode because in Select mode you would want your typed text to appear literally as if you were in Insert mode.