I've tried to replace the bg of a video by using diffrence masking, I've followed this guide Difference Masking-Unmasked in Sony Vegas Software
I've done this once before and it worked but not this time for some reason, I've exacly as they did in the guide. Here's a pic of the result ImageShack - Hosting :: fail2ky1.jpg

What I wanted to do was make it look like the car was flying but the bg didn't change, But a little bit of the car changed some how (The bluer part in the middle).