first it helps to know that character models are saved in the same file as the skins...
a vertex means someone has directly edited the model included with the base skin to look however they want...
an import, on the other hand, is a skin where someone has imported a model from a different game or something...
both vertex and import skins are applied to PM like regular skins... you may consider using only those marked wifi-safe, though... cause those that aren't might have edited bones or something that might cause things like hitboxes apearing in the wrong places and so on...