FixVTS is a tiny tool that allows you to adjust your DVD files into better DVD compliance, so they can be opened in DVDShrink or vobblanker. It does a similar job as what's called an "IFOEdit Mock Strip", but only adjust as little information as possible in the VOB and IFO files to allow opening in other apps while keeping the DVD as similar as the original as possible. You would use FixVTS if you messed up your DVD, for example by applying the wrong tool, or aborting in the middle of some processing, and the result can no longer be used.

Another good use is for DVDs that have been authored with errors in the IFO or VOB files, such that they still play on your player, but won't open in DVD editing tools such as DVDShrink, vobblanker and others.