An explanation of what FixVTS does:
FixVTS opens a full Video Title Set (i.e. a menu vob, or a series of title vobs) and updates a number of important pointers in the vob file. Specifically, the forward pointers (enabling to quickly go from navpack to navpack) and the two this->LBA pointers that indicate the sector of the current navpack.
In addition, FixVTS keeps track of all the navpacks, and of all the cells in the vob file, and adjusts the IFO file for the titleset so that they match what's in the vob files.
FixVTS can also remove unreferenced cells (i.e. cells that are in the vob files, but never played back, according to the IFO file).
FixVTS also fixes blank packs that are sometimes introduces by DVD rippers when they encounter unreadable sectors. These blank packs can either be removed, or replaced by compliant "stuffing packs".
FixVTS can be compared to an IFOEdit "Mock Strip" in the sense that it performs some of the modifications that IFOEdit does. However, FixVTS does not update all the pointers IFOEdit does, and does not mess up the PGC cell flags or the cell commands. It does reorder the VCIDs, which is something that a mock strip does not do. In addition, it is significantly faster (in-place mode) and can process an entire DVD with 1 click.
FixVTS tries hard to process as fast as possible by re-writing only what's necessary. When processing from backup, everything must be re-written, so that's the slowest option. When processing in-place, and if the option to remove unreferenced material is not checked, only blank packs and navpacks are re-written, and only if it's necessary (for example, if some of the pointers are wrong, or if they are renumbered).
It won't transcode the video stream.