I just brought up a 2008r2 64 bit server in the my 3 host (6 CPU) config. This machine is slow as heck, I know doubling the RAM will help but what can I do to maximize speed in the CPU area? Virtual sockets and cores are both currently set to 1 and yes the CPU utilization is pegged at 100% These are pretty new Dell R710 hosts but I just looked at the configuration in vSphere and it tells me processors sockets = 1, cores per socket=4 and logical processors=8.
You can add a second vCPU to a 2008 R2 VM by shutting it down and making those changes. Windows itself will need no additional changes made. With the host processor setup you describe, this will be fine.
Just out of curiosity - What is consuming the CPU? I have many many 2008 R2 single vCPU VMs that have no issues.
You can try to hot add cpus, if you cant afford downtime. Before that while the vms are running, look at esxtop,and select V to see the VMS utilization of cpu. This is to confirm that no other process is hogging the cpu