As workaround solution VSS-Cisco and VLT - Dell should live together. Someone proofed this solution and worked fine? In this way like 2x6500 works like unique only would exist one LACP configured in Dell site.
I don't have any documentation showing VLT switches connecting to VSS switches. But here is a white paper covering MLAG. It has some examples of MLAG connecting to VLT, and Cisco switches.
The concepts will be the same for connecting Cisco to the Force10 switches.