What's what's this fan patch and why do I need it?
I own the vanilla steam version
https://www.moddb.com/mods/vtmb-unofficial-patch/downloads/vtmbup103
There's where to download it. It mostly fixes bugs and some crucial fixes to the game, it's balance, and some game breaking bugs. That's the main reason, fixes a lot of crashes, progress bugs, etc.
However, when installing there is an optional choice for a "plus" patch (which is turned off by default, but can be selected when installing) which add in several scrapped areas and missions from the base game that were cut due to time and re-implement them fully functioning (and include a few more controversial "fixes" to the game for playability reasons).