Hi Jon -
I'm sure someone may have a much cleverer way of doing it, but I use the method outlined in this document to alert on this sort of thing. I've got a similar set up as yourself and I've simply created a custom property to tag the VSS VSL and fast-hello interfaces as "interesting" and hence to send an alert when one goes down etc.
The method works the same for 3750 stacks, but it's just a case of assigning a custom property to the stack ports.
Thanks John, just implemented your doc and it works a treat.