You will also need to use Routing->Rules instead of firewall mark-routing to be compatible with FAST-TRACKING
For a resource-intensive script, See also; http://forum.mikrotik.com/t/very-slow-pptp-tunnel/142282/9
You may also choose to route all traffic or selected prefixes via OPENVPN, using Routing Rule(s), statically, to save from expensive router resources..