VMS 2.3 which includes IPS MC 2.1 can manage 5.0 sensors (not 5.1).
Once you upgrade the VMS to VMS 2.3, upgrade the IPS MC 2.1 separately to IPS MC 2.2 separately.
You can find VMS 2.3 here. (need CCO login)
http://www.cisco.com/kobayashi/sw-center/cw2000/vms-planner.shtml
It will require your existing VMS 2.x license.
Please note that VMS is end of sale. If you are planning to use IPS 6.x, please consider CSM (Cisco Security Manager).
Thank you.
Edward