Is the LinkStar a download only DVB service or is it a full two way service in itself?
I have worked with both of those services, and as you have found out they each have their advantages and disadvantages...
The problem you face is how, when you send a request out via one gateway, does the reply to that request come back through a different one. Thats why I ask about the DVB service.....
So, you can do some polciy based load balancing that will probably help. Here are some example policies.
So: Traffic that is heavy on upload goes IN AND OUT IDirect:
PPTP VPN (But VPN sucks on Vsat anyway as does VOIP)
And Traffic that is heavy on download goes IN AND OUT Linkstar
For details about how to do policy based routing look in the Mikrotik Wiki, in the routing category http://wiki.mikrotik.com
Its not exactly what you want to do but you dont have much choice.
Does anyone else have a suggestion?