I think your question is flawed. The walls of the vagina (btw, not the inside of it, because the vagina is the 'inside') are mucous membranes, not 'considered to be', they are. Period.
Mucous membranes are not skin, and skin is not a mucous membrane, they are two separate things and the walls of the vagina just happen to be mucous membranes (just like the inside of the mouth) while the labia minora happen to be made of skin.
Maybe you should look up what is a mucous membrane?
The labia minora is skin folds that do NOT produce mucous, so they are just skin folds. The vagina walls produce mucous. I guess a good example would be when you and your boyfriend are kissing and getting ready to make love, you "get wet". BUT the labia manora does NOT get wet, the vagina walls get wet. Make sense?