What I have done in the past is copy the VOB file (VTS_01_1.vob)from the DVD to a file folder on my computer.
Open Sony Vegas (I’m using 7) and in the explore tab locate the folder I just copied the .cob file to.
Then click on the Views button in the tool bar just below the envelope tool button and make sure the all files box is checked.
When it is the .vob file will show up and just drag it to the video track and edit.