Going to deinterlace options in Vegas:

File -> Properties -> (Project properties) -> Deinterlace method

There are three choices

1. None
2. Blend Fields
3. Interpolate fields

“None” is what I have but what is and when would you use 2 or 3 ? What I do know is that if 2 or 3 are selected (I did this accidentally with 2) Vegas gives an error message and stops rendering on a flash transition just at the point where the screen goes white. Re F

Blend would be used when you have  interlace clips and you want to render them progressive..

 or if you notice lots of interlace artifacts ( changing clip speed, scaling clip size ,etc ) and the "reduce flicker"  doesn't do it for ya ...