There's no link between the ''Preview Window'' which is where you see your video from the TIMELINE and the ''Rendering part''.
The reason why the ''Preview Window'' can be set to ''Draft'', ''Preview'', ''Good'' and ''Best'', is because some computers might have problems with viewing the edited footage if its very high quality. It might turn it to make heavy lagging, if your computer is a bit slow, and you try to run the HD footage. So to solve this, you can simply just turn the Preview Window to ''Draft - Half'' and that should solve the lag in vegas.
No matter how good or bad you set the Preview Window, it WON'T have ANY effect on the output video quality. It's the way of Rendering your footage which matters, regarding the quality of the video

So if your vegas lags: Put the Preview Window on ''Draft - Half''. Hope it helps
