sounds like it could be sampling rather than interlace problem - try rclicking on clip, properties, video event, disable resample.

I just had the same pickle to work out. I forgot the setting on the camera was 24f and filmed an event for TV. The station needs interlaced footage in Quicktime.
In vegas it's easy. Go to Options, Preferences, General, and clear the 'Allow Pulldown Removal when opening 24p DV' tab. Now Vegas reads the footage as interlaced.
Put the footage on the time line.
Click File, Properties and Project properties pops up.
Use the Template NTSC DV
Lower field first, no de-interlace.
Then render to Quicktime and use the Sorenson3, and lower field first on the video options.
Anyways it worked for me and they're airing the footage tonight. Hopefully it looks good. They originally wanted the quicktime rendered in the DV/DVC Pro-NTSC option but it looked like crud so I tried the Sorenson3 and it looks great.