Hello Dom,
>> There is no restriction from the VSS to use DFC3C / DFC3CXL. Similar to normal switch requirements, either line cards with DFC3C/CXL can be used, or only the CFC can be used.
I read it as: or you use DFC3C or DFC3CXL OR you have the option to install a CFC and to use the linecard in centralized CEF mode.
See also config guide
PFC, DFC, and CFC Requirements
The VSS supports any 67xx series switching module with CFC hardware.
The VSS supports DFC3C or DFC3CXL hardware and does not support DFC3A/3B/3BXL hardware.
If any switching module in the VSS is provisioned with DFC3C, the whole VSS must operate in PFC3C mode. If a 67xx series switching module with a DFC3A/3B/3BXL is inserted in the chassis of a VSS, the module will remain unpowered, because VSS supports only DFC3C and DFC3CXL.
If the supervisor engines are provisioned with PFC3C, the VSS will automatically operate in 3C mode, even if some of the modules are 3CXL. However, if the supervisor engines are provisioned with PFC3CXL, but some of the modules are 3C, you need to configure the VSS to operate in 3C mode. The platform hardware vsl pfc mode pfc3c configuration command sets the system to operate in 3C mode after the next restart. See the "SSO Dependencies" section for further details about this command.
http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/vss.html#wp1053927
So yes only DFC3C/DFC3CXL for distributed CEF mode or CFC for centralized CEF mode
Hope to help
Giuseppe
