Vlan Group

Not an expert, but in the absence of a more authoritative response, a VLAN group is synonymous with a layer 2 bridge between two VLANs in the same layer 3 subnet space. Per the support site, "VLAN groups are intended for load balancing traffic in a Layer 2 network, when you want to minimize the reconfiguration of hosts on that network. A VLAN group also ensures that the BIG-IP system can process traffic successfully between a client and server when the two hosts reside in the same address space" and is another way of avoiding SNAT.