Per Ben Canning:
Yes, you want each VIP to have separate VHIDs. We use VHID 10 and VHID 20 internally to communicate between the controllers so I recommend using numbers higher than 20 for your VHIDs. Critical groups only apply to individual systems so you can have Critical Group 1 (for example) set up on multiple systems without affecting each other.
The critical group works this way: if every interface in a critical group goes down, a failover occurs. If you have just one interface in the group, failover will occur if it goes down. If you have multiple interfaces in the same group, all of them must go down for a failover to occur. If you want a failover to happen when any interface goes down, mark them all as critical and put them in separate groups.