I've looked around, i think the VBI option is only there for tuner cards. My video is coming in off a firewire card connected to a cable box.
I do see audio sync problems with SD recording, strange the HD stuff works fine. Other than mash and kids shows we don't watch much SD, so it hasn't been a big deal.
Although I don't have a frontend for the myth box, I did pull a video onto my laptop and it played fine.