I’m not sure he can keep the UV’s from the torso on the vest 100% (though he can preserve the original model.) The vest would have thickness and a back side to it that would represent new 3D topology which would infer new mapping in the UV space. An alternate approach would be to dupe the model, hide everything but the vest and use “delete hidden” to preserve just the vest shape as a shell.
The shell should retain the UV space already mapped. I haven’t tried this, but I believe you could then store this as a morph target, slightly inflate the shell and use “create morphdiff” to generate a thickened piece which would retain the UV space on the front, though the sides and back might be hosed until you run it through a UV processor like UV layout.
Personally, I’d just make the vest with extract and UV from scratch since the “cuts” prior to flatting into the UV space are pretty straight forward. My main concern to would be to preserve the torso’s UV’s so I didn’t mess up any texturing that may be have been done there.
-K