Configure Site-to-Site VPN as automatic Point-to-Point WAN failover?

[lxzndr]

Want to set up Fortigate to Fortigate VPN to use as a failover route for our Point to Point routed through our existing Cisco routers.

Cisco routers are the computer’s default route address, the Cisco router has EIGRP to learn the other side addresses, plus has hard routes for the Fortinet Wireless networks, and DMZ network.  Cisco default route is the Fortigate.  Fortigate also has routes for the internal networks that point to the Cisco Routers.

would like for this to automatically route WAN traffic through VPN when WAN link is down, then return to the WAN link when it comes back up.

What is the best way to set that up?  

currently cisco 2821 routers at both ends (those will be upgraded in the not too distant future)

and Fortigate 100D (5.2.7) at both ends. 

(if it matters for configurations, we have Cisco IP phone system)

Thank you,

Mike