Hi:
I haven't dealt with VSS, but I think your question speaks to a more common concern: whether to create a routed interface or place a switched interface in a vlan to create a point-to-point routed connection.
I always prefer the former if I am using IOS. With CatOS, you dont have a choice but to do the latter.
I would execute the no switchport command on the ethernet interface on the 6500, apply an IP address, and be done with it.
HTH
Victor