Hi Craig,
From this White Paper on VSS -
"Because a VSH (virtual switch header) is appended onto every frame that is sent across the VSL, there are also new port ASIC requirements to be able to form the VSL. The interfaces that can currently form the VSL are the 10 Gigabit Ethernet uplink interfaces on the Cisco Catalyst 6500 Series Supervisor Engine 720-10G VSS as well as those on the 8-port 10 Gigabit Ethernet line card (WS-X6708-10G-3C/XL)"
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps9336/prod_white_paper0900aecd806ee2ed.html
So upgrading the 6704 is not going to help and a 6708 line card would be required if you want to have a third VSL.
-> Sushil