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
If you are logging the above error your /etc/pam.d/proftpd was updated (incorrectly)…