Since a VSS is a single system image, does it make sense to enable UDLD for links between the switch pair? We've seen some recovery problems that we feel might be due to UDLD mistakenly identifying an ISL link as unidirectional, especially in a configuration with ASA-SMs in an HA configuration.
If you are referring the the VSL between the to VSS switches, I don't think it makes sense to enable UDLD because LMP will detect a unidirectional link.
LMP is designed to run on each VSL member link to maintain multiple state machines with the same peer over different ports. In a case in which a unidirectional condition on a port is detected, LMP marks it as down and attempts to restart VSLP negotiation—instead of err-disabling the port.
For this and more LMP and VSS info: http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/VSS30dg/VSS-dg_ch2.html#wp1055977