It's actually not. DVD is what you end up with, not what you start out with. If you want to reedit a project, go back to the original media. Don't start out with what you ended up with last time.
Unfortunately I'm not longer have the original media on my system, will I have to upload the footage and re-capture everything again? is there any way I can do this by extracting the VOB files and converting them in the correct format to re-edit?
OK, as of CS4.1, PrPro has the capability to Import 100% DVD-compliant .VOB's. You state that your .VOB was created by by PrPro. In CS4, this is not possible. As of CS3, the DVD's MUST be created by Encore. Is this what you mean? If so, they will be 100% DVD-compliant, so the .VOB's will Import fine, and edit as well as an MPEG-2 can.
Take a look at this ARTICLE for background on .VOB's.
Now, with a 100% DVD-compliant set of VOB's, you should not have an OOS issue. Take a look at this ARTICLE for ways to correct OOS issues.
will I have to upload the footage and re-capture everything again?
"Upload" typically refers to transferring a file from a local computer to the Internet. "Capturing" is the term used for transferring from tape to hard drive. And yes, recapture is the correct method here.