You are really just lacking fail-over capabilities. The scale is more about how many vCenters you attach to the vCloud Director application, and how many concurrent users (web and console) you might have at a given time.
But if you're deploying a RHEL (or CentOS) VM, is there any reason why you wouldn't just make 2? It would have a requirement of an NFS share as a loose cluster drive, and possibly a load balancer (or just use DNS round robin).
The distribution of load is much nicer, especially if you have multiple vCenters to be added.
The thing you'll want to look at is the performance tuning white paper : http://www.vmware.com/files/pdf/techpaper/VMware-vCloud-Director51-Perf.pdf
this paper should cover most/everything you should need.