You can run multiple (up to 10 with SRM 5.1) Recovery Plans simultaneously as long as they don't contain same Protection Group.
I can see certain complexity here:
Let's say you have vCenter/SRM VMs at site A and they manage/protect Sites B and C too.
1. When Site A goes down, sites B and C are unmanageable untill you bring those vCenter / SRM up at site D. If you have streched VLAN, this is relatively easy. But if you have to change vCenter and SRM IP addresses, this will require a lot of manual work.
2. If your vCenter / SRM VMs are protected, then you are not able to perform planned migration of recovery plan containing those VMs.
This will affect you in two cases:
Planned Migration of Site A to D
Failback from D to A.
As a workaround, you may put vCenter and SRM VMs on separate replicated LUN and failover / failback those VMs manually.
Michael.