migration from Virtualmin to Centminmode

No experience with Virtualmin myself, maybe others can chime in.

Though migrations are almost always the same way see the below guides for general gist of any type of server data transfers/migrations

For moving site data there's 2 general guides where the principles & steps of backing up and moving data is the same just slightly different for

1. Moving Centmin Mod site data to another Centmin Mod server
2. Moving cPanel/WHM site data to Centmin Mod server

Those guides move alot more than just the site(s) in question but config files etc. You can use your own best judgement as to the config files and whether you want to move them or just use the fresh server setup Centmin Mod config files as they get auto optimised for the server Centmin Mod initially installed on.

If your old site isn't centmin mod or cpanel/whm based, then those guides won't be 100% applicable. But the general overview of what needs moving and where everything is structured in Centmin Mod LEMP environment is worth reading up on.

Also official site configuration files page explains the Centmin Mod structure of where the important files are so you can decide on what to backup.