When you import clothes into vam, the cloth item is wrapped to the vam model based on the cloth mesh to the nearest vertice on the model. For most of the body this is fine and doesn't lead to much distortion of the cloth but the crotch area is well known for having issues.
There's a couple of plugins that can help this, Skynet's cloth sculpt plugin and Regguise made a plugin that allows you to change what vam wraps the cloth to. Worth giving these a go!