Switching QMail+Vpopmail to Postfix Pl share experience if two MTA can coexist

I don't have any familiarity with vpopmail, so I don't really know how to guide you on that. I think a backup and then restore onto a new system running Postfix (which is the default configuration in Virtualmin) would convert it for you. That gives you the benefit of being able to keep the old server online while you test your backup/restore process and test the resulting configuration and mailboxes to make sure they're the way you like them before you flip the switch on the migration.

Making it in-place seems like more trouble than it's worth, but if I absolutely had to do it, here's some things to think about when making any mail server change:

1. Where and how is mail stored? The VIrtualmin default for Postfix+procmail+dovecot is Maildir in the users home directory, under the Maildir directory. If your current mail config isn't storing mail in that format or that location, it'll have to be converted and moved. I think vpopmail has a central mail spool, but I think it does use Maildir, so just copying into place would probably work.
2. How domain to user mapping happens. In Postfix, we use the virtual map file(in /etc/postfix/virtual. You'll need to set that up. I dunno if there's an automated tool for converting from whatever vpopmail does for this purpose...it's probably easily scriptable, though. The virtual map file in Postfix is a super simple key<tab> value list.
3. As I understand it vpopmail has mailboxes without a system user associated with them, so you're going to need to create system users for all of those mailboxes. If this is a big mail system with a lot of users, you likely need to automate this. You probably want to write a script for this.
4. Switching the Virtualmin config in-place is probably going to be tricky. I don't actually know what bits need to change to tell it to use the new config...there's a lot of moving parts in the mail system.

The more I think about it, the more I think you really ought to migrate to another system, instead of trying to switch in place. Unless you can afford some downtown (on the order of hours or days), it's not worth even attempting it, I don't think. Mail is among the most complicated parts of a Virtualmin system (Postfix, saslauthd, dovecot, procmail, clamav, spamassassin, Usermin, Webmin, Virtualmin, all have configuration options related to mail and all have some sort of metadata and dependencies that have to be working together for things to look cohesive). It's gonna be a mess if you're not really comfortable with configuring and troubleshooting both mail configurations.