Well, normally you would achieve that by going into Limits and Validation -> FTP Directory Restrictions, enabling a restriction for user home directories.
Then, create a new FTP user by going into Edit Mail and FTP Users -> Add a website FTP access user.
That would create a user whose home directory is your DocumentRoot.
Would that do what you want, or did you really want that to work for the Virtual Server owner?
Well, Virtualmin and ProFTPd should work just fine on a shared IP address.
Any user with an account on your server should be able to enter their domain name as the FTP host, and it will connect them to the ProFTPd instance running on your server.
There’s only one user created by default though, that’s the Virtual Server owner. If you want additional users created, you’d have to do that by hand.