I'm in the same boat. I've been trying many different authoring software, but none would take mp4 directly. However, I THINK I just found what both you and I need. Roxio Video Lab will let me author the DVD and all that good stuff, and from what I can tell, it will allow you to split and export whatever sections you want.
downloaded the software, tested it, and it doesn't support these kind of files. Neither audio or video are being recognised (Vegas at least recognised the audio...)