You cannot cut .vob files off like that. The structure of dvd-video doesn't allow this.
What kind of files are you getting from the Dazzle unit? Are they already in dvd-video format, or are they avi or mpeg2 perhaps? You really want to split the video before putting them into dvd-video format if possible.
If you can capture in avi format, then split them with AviDemux or Virtualdub. You can split the mpeg2 files with AviDemux also (though I don't believe it is optimal for this) or you can use the free trial of VideoReDo TV Suite, which is a frame accurate mpeg editor.
If you want to work with the dvd-video you have now, you can cut the video in two with DVDShrink in ReAuthor mode, using the Start/Stop Frame tool. You select the first section by hand, save it as a complete dvd video, then go back and select the second section and save it. You lose menus using this method.
If you want menus on both, it would be better to work with the video before putting them in dvd-video format. Having said that, it is possible to cut them in the trial of VideoReDo TV Suite, then work with each section to make new menus...so that is a possible solution for you if all you have is dvd-video. Nero might be able to do this also, but I don't trust it not to reencode and lose quality.
Now, going back to your original problem, which is skipping towards the end of the movie. That is a classic symptom of poor quality media and/or burning too fast. I suggest getting some Verbatim 16x disks and burning them at 8x speed. See if that helps.