why proftpd instead of vsfptd?

No particular reason. We had to pick one, and ProFTPd already had a good Webmin module (vsftpd is now also supported via an extra module) and is more widely used in virtual hosting environments. I actually prefer vsftpd, due to its simplicity and better security history, but ProFTPd is also a good server and the security record has been excellent in the past couple of years.

If you have a strong preference, you can choose to switch to vsftpd. I'm working on giving the installer the possibility of allowing folks to make some choices during installation, as well...so in the not too distant future, you'll be able to choose between vsftpd and ProFTPd, and sendmail and Postfix, during installation. At the moment, you have to switch manually after installation, but that's not too troublesome (unless you do it after you've started creating domains).