Theoretically, if the controller at each site is providing DHCP for each individual site, that AP will automatically connect to that local controller, either using the DHCP option that is automatically inserted or a layer 2 broadcast if you do NOT have an LMS-IP in the AP-group (ap system profile to be exact) of that AP. So in theory, you can make it the same ap-group for all.
Your second consideration is that each virtual AP has a VLAN in it, and that might or might NOT be relevant locally in the same ap-group. What you can do, is use named VLANs in your virtual AP and then define the VLAN name to VLAN mapping locally at each controller. In that way, each user will connect to a Virtual AP, but the VLAN will be locally significant.
I hope this makes sense.