Enable VPN for Interface directly

I have a VPN setup, as detailed in the following instructions:
https://www.privateinternetaccess.com/helpdesk/guides/other-hardware/mikrotik/mikrotik-pptp-2

This creates a PPTP interface, which then routes to 0.0.0.0/0 and so all traffic that is put through that interface - via address group lists in Firewall - gets ‘encrypted’ and sent to come out via an anonymous egress point. The issue is that it just uses the default gateway of the Mikrotik. I need to direct it…

However, I have 2 WAN connections, to two different ISP’s. What I would like to do is the following:
Have all traffic that is destined for one interface, automatically send all traffic inside the PPTP tunnel.

So in essence, in the before mentioned link, instead of having the PPTP gateway forward to 0.0.0.0/0 I would like it to go to WAN2, which will then forward to 0.0.0.0/0 via a “2” distance.
I am sure this is just a more complex routing thing, but has anyone does this? My iterations have not given expected results. Especially as TraceRoute does not help, as it puts the traffic inside the runnel at the first hop, then I can only view interfaces to try and see if one is being used more than the other:-)