They aren't always 10 second clips, but more associated with what's happening on the timeline and convenient breaks. You can see this by applying longer or shorter clips and images on the timeline.
Vegas breaks it up for a couple reasons so far as I know (or don't know) but it's related to making edits and not losing the entire pre-render. Another benefit is smaller prerenders are fragmented less on the drives, so it doesn't choke up in playback on slower systems.
i see. is there a way for vegas to capture from mini-dv without regioning out clips to seperate files - to just capture one entire piece with all the edits in one file?