Hi Bryan,
VPN monitoring does this job. Enable VPN monitoring and Rekey options on all IKEs. If VPN monitor fails because of failed SA the NHTB entry which maps the SA to the VPN route change it's status to Down and deactivate the associated VPN route. The NHTB entries are generated dynamically if both devices are SSGs. Otherwise you should configure them manually. I usually use the remote GW IP both as the next hop in the VPN route and as the NHTB entry. If remote GW has a numbered tunnel interface I use it's IP instead. The dynamically created NHTB entries are generated by ScreenOS in accordance with the described scheme.
I also recommend to enable VPN monitoring option "Optimized". This will prevent route flapping under a havy VPN traffic load.