In FWSM you can assign any number of VLAN's to a VLAN group and then assign this group to the FWSM. You cannot assign the same VLAN to multiple firewall groups; however, you can assign multiple firewall groups to an FWSM and you can assign a single firewall group to multiple FWSMs. The FWSM checks for the VLAN ID of the traffic to realise to which particular VLAN it belongs to.