Does the login to your newly installed CentOS 6 VPS using Virtualmin and ProFTPD fail?
Check your /var/log/secure for errors such as this :
proftpd: PAM unable to dlopen(/lib64/security/pam_stack.so): /lib64/security/pam_stack.so: cannot open shared object file: No such file or directory
proftpd: PAM adding faulty module: /lib64/security/pam_stack.so
proftpd: 192.168.0.1 (192.168.0.155[192.168.0.155]) - USER mynewdomain (Login failed): Incorrect password.
If you are logging the above error your /etc/pam.d/proftpd was updated (incorrectly) and you need to replace it with:
auth required pam_listfile.so item=user sense=deny file=/etc/ftpusers onerr=succeed
auth required pam_shells.so
auth include system-auth
account include system-auth
#session include system-auth
session required pam_loginuid.so
The above change is tested and works on CentOS 6.2 running Virtualmin 1.585.
Edit: This is still valid for CentOS 6.4 running Virtualmin 1.650.