vpn failover for wireless bridge connection

I have a seperate warehouse that we connect to with a wireless bridge at 56mb on the same subnet, for voip, and data entry. We also have a backup server, that we dump our main servers to at night.

At night the bridged connection is stable, but during the day we have dropouts do to Large trucks temporarily blocking the line of sight. We are restricted by antenna heights.

We also have a DSL connection at the warehouse, so if the main facility goes offline, remote offices can connect to the backup location and remain operational.

The main office runs on a Linksys RV042, that I would like to connect to with a VPN and failover to that connection, when the bridge goes down.

Any hardware recommendations or solutions?