Are you using any routing protocols?
You could use floating static routes..
ip route 10.10.10.0 255.255.255.0 172.16.24.1
ip route 10.10.10.0 255.255.255.0 192.168.1.5 200
This sets the route via the VPN (192.168.1.5) with a higher AD so it will be used in the event the route to 172.16.24.0 goes down.
Hope this helps.