>Attempt to add an underwear version to my avatar so I can have an undies version if I want

This is...exceedingly more difficult than I expected. There are so many issues with the weight painting that I can't even begin to describe it. If I move the legs it completely clips the front of the undies all over the place and the back does not clip enough so it doesn't stick to the buttock. On top of all that the bottom half "sticks" to the thighs so it looks fucking dumb when I move the legs to the left or right. 

Normally I just cheat with normal clothes by just deleting all the vertexes under the clothes because you won't be seeing them anyway and this removes any issues with clipping, but because undies are small you can't really delete things below it without just creating gaps in the mesh or exposing empty mesh space when you move the legs.

How on earth do I achieve this? Is there any quick cheat way to just add these undies properly weight painted to the model? I'm not seeing how I can do it manually like this