Policy based routing takes effect before the virtual router has a chance to do any routing on the traffic. For the monitor, choose the IP address of the ISP gateway. In your virtual router, put in a static route for that gateway as a /32 address out the proper interface.
Static Router WAN1 Gateway:
This forces all the traffic destined for the WAN1 gateway out that interface. Then use that WAN1 gateway IP for your monitor IP in the PBF. This way if that IP is unreachable, PBF policy will disable its self. Since its a /32 address, general routing priciples tell the system that its a more specific route than the default so the default route will never get used to get to the WAN1 IP address.
Similar for WAN2.
Hope this makes sense.