Vegas won't recompress if the stream is DV AVI, some other types of AVI, and HDV m2t. It will recompress with all other formats, or if you used transitions/pan/crop/plugins/wrong I/O settings.
If you just want to re-wrap a video to a different container, your best bet is ffmpeg or Quicktime Pro (for h.264 MOV to MP4), not Vegas.