Is there any way you could post screencaps to better illustrate what is happening? I have the pdi card and I have had problems too, the video image is shifted up in the capture. My audio professor said it might be a timing issue.
http://forum.videohelp.com/topic335815.html

But that's not the same thing you're seeing, right? I've never noticed anything wrong with the fields in Sony Vegas. Even if I switch between even & odd fields first in the preferences or render box. But then again I haven't worked much with the pdi card so far. Either way, I have got to believe that there is some kind of control or plugin to compensate at the capture stage so you don't have to average it out in avisynth.