Multi-homed ISP connections for VPN

I have a LC that is multi homed to two ISP's (ISP A and ISP B). Internally I have a VPN concentrator that is defined in the Wide IP. I would like users to primarily connect through ISP A,a nd only fall back to ISP B if the link for ISP A is down.

 

 

How do I set up the pools, virtual server, and LB method for this to work sweetly.