The FWSMs are not VRF aware. So you would have to have two interfaces off the FWSM to route between the different VRFs.
Each VRF should have a transit to an interface in the FWSM. From there you need to setup static or dynamic routing on the FWSM to peer / route into the VRFs.
The final piece is to ensure that you have the proper ACLs to allow communication between the interfaces, depending on the security levels.