Multiply WAN routing problem.

I have a CCR with two ISP failover config.
ISP 1 is prio och ISP2 is failover.

My problem: When I connect with VPN/PPTP I want my traffic to only use ISP2 interface.

How?

In the mangle add a rule for mark routing for this traffic and then add a static route for this mark with gateway ip from isp2