You might want to take a look at the instructions Linode offers for
installing Zimbra on CentOS. It is pretty straightforward, even if I still have an issue with getting the logger to perform correctly after install. Also, I suspect that most people want to use their "hostname -d" (eg. domain.com) rather than "hostname -f" (eg. mail.domain.com) for email addresses.
Depending on how you installed CentOS or the specifics of the image from your VPS you may need to add different packages, too. I'm personally pretty happy with Linode, because they make it extremely easy to resize nodes and change the capacity to fit changing needs.
-Randall
Note that CentOS is meant to be identical to RHEL. The replication of functionality is even said to be bug-for-bug compatible.