Hi,
I think ip base supports VSS. For 6500 you can't purchase licenses. It works based on the feature set.
So I believe if you have ip base thats the minimum requirement you have.
Have a check for VSS software and hardware requirement below
http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-6500-virtual-switching-system-1440/prod_qas0900aecd806ed74b.html
Hope this helps.
Thanks,
Madhu