Shooting in PAL means you are presenting Vegas WITH lower field first (LFF) material - PERIOD!
Make sure your settings throughout are for LFF. Why you should be getting "better" results with anything OTHER does not make sense. I can't argue/comment on your perception of "better", other than to say it is NOT that which is within the scope of the PAL strictures. "Better" might mean you are getting an "affect" as a result of field reversal 'cos of some other reason. And as for NONE, which is for progressive footage then I invite others comment.
I am all for experimentation, it is at the heart of what we do! But "better"? When you say you are in PAL? Interesting.