Rigging issue, legs weren't in the right position when you copied weights from FNV mesh, they were sticking more to the left and right. Since you binded mesh to a skeleton without actually matching how skeleton is positioned it created this "bug". Skeleton is still in same default pose as in your 2nd picture (vault suit) but mesh is sticking out of the skeleton and is still behaving as if It's properly placed just following how the skeleton is moving thus giving you that stance. Try to position your custom mesh as close as possible to FNV mesh and then just copy weights. That being said this can be done intentionally on bulky meshes where it makes sense for that model to walk a bit wider so legs don't clip (assuming you don't wanna edit model to match FNV body proportions but wanna keep original model proportions), on regular skinny humanoids it just looks as if they shat themselves.