From what I can tell from the peri, if there's going to be sagging, it's going to happen regardless of a vest. Skin usually tightens over time when you've lost mass from any part of your body, anyway. Make sure you keep hydrated with plenty of water and take a multi vitamin. As long as your skin isn't too stretched out already (which, since it was a peri, I imagine it wasn't), you should be fine. Try not to worry too much about compression vests.
The major reason I'm aware of to wear compression vests is to cut down on post- op swelling and bleeding, basically keeping pressure on the wound beneath the skin. They tend to do them in DI because they tear the flap of skin up during surgery. Dr. Medalie has a video of himself doing both a peri and a DI on his YouTube channel (DEFINITELY NSFW and NOT for the squeamish. If you can't handle blood, I recommend you NOT to seek them out) and the differences are really interesting, though both do leave what basically amounts to a hollow in your chest under the skin. From what little I know of medical science, there's more trauma at the site with DI than with peri.
As for why the other guys got one, I don't know. Maybe there was more bleeding or he's more prone to swelling? There could be a lot of reasons, so I wouldn't worry too much.
