Note that when you actually buy a VPS, nothing happens to the hosting account automatically. You just get a VPS alongside. If you want to move everything to the VPS as a replacement for the Plesk hosting account, you would need to do so manually, then cancel the hosting account.
Some of our VPS users do have both, they typically use the Plesk account for their main website, and set up the VPS on a subdomain to use for hosting large files or running a specific a heavy application like a game server.