I use an Avisynth filter QTGMC, considered by many to be the best, and much better than Yadif (although Yadif is an option) and certainly better than those inside of Vegas. It does require installation of Avisynth and Virtualdub and a script to control the round-trip processing from Vegas and back. In the past, I’ve used a single-purpose script to do this but have recently created a GUI version that can easily accomplish this with just a few clicks. It adds the de-interlaced file as a Take to the original event. It should be available in the not too distant future since it’s undergoing pretty much final testing. It also supports Deshaker for those interested in a freeware alternative to Mercalli.
wwaag