Wayne is right when he says to just rip it to your hard drive.
Copy the contents of the DVD to your hard drive.
If it only has one VOB file, drag and drop that onto your Vegas timeline.
If there’s more than one VOB file, here’s a great technique to avoid the audio dropout that alwyas happens at the change point.
Vegas allows you to open up a DVD .IFO file. This imports the entire .VOB chain for both video and audio tracks.
Click File / Open, then navigate to your VIDEO_TS sub directory and enter *.IFO in the File name input field. For a regular DVD you will see a VIDEO_TS.IFO, and a VTS_01_0.IFO. Select the VTS_01_0.IFO file and click on Open.
When you browse to the VIDEO_TS folder using “Import Media”, it will appear to be empty until you select “All Files (*.*) in the import box. Once you do that, all the files will be visible. Select the appropriate one, import it and drag it to your timeline.
