I have resolved the issue. This is what I did:
This is 5.1.1.0 VSA configuration.
Error message: Cannot create VSA cluster: Cluster's subnet is invalid. Should not be'
Please start with this article and resolve all. Ensure your gateway is pingable and your ports are trunked.
http://kb.vmware.com/kb/2004832
Note: this is a brownfield installation with live virtual machines running. No downtime whatsoever. When format disks prompt appears after network configuration going into the VSA cluster configuration, this does NOT overwrite the virtual machines but instead carves out the remaining free space on the local datastores. As you storage vmotion virtual machines off of the local datastore, more space will be available to add towards the VSA datastores. If your vcenter is virtual, which is not a supported configuration, DO NOT move this machine off of the local datastore that it is on. In other words, don't move vcenter to the VSA datastores.
Note 2: Ensure your port groups names are spelled and spaced correctly as shown below. This is exact. If not, the install will fail.
2 node cluster setup
vSwitch0 - port groups
VM Network (vlan 13)
VSA-Front End (vlan 13)
Management Network (vlan 13)
vSwitch1 - port groups
VSA-Back End (vlan 10)
VSA-VMotion (vlan13)