Got your own content want to re-edit quick-like using iMovie. Use MPEG Streamclip to export to MPEG-4. Import into iMovie. Edit.
If you’ve got a bunch of VOB files on a DVD of non-copy protected stuff (home movie telecine etc) use the same tool to convert the DVD into a AVI or other format file. Don’t forget that if you’re on a Mac you may want to watch any resultant AVI using VLC!