The Unofficial Patch is not really a mod, but while the default basic patch only fixes bugs, the optional plus patch restores a lot of hidden content, including new quests and new maps!
To my knowledge Dr. Werner "Wesp5" Spahl has focused on restoring unused, or accidentally flawed, parts of the original so-called vanilla version of the game.
Patch 8.8 includes a long list of achievements in this restoration & preserving of original storyplay & atmosphere. The patch comes in a size of below 300 MB and still manages to bring significant improvements to the entire game.
Certain fan-wishes, logic-inconsistencies, and other weirdness have been solved, too. Even a chance to join the Sabbat and see the ending film of that choice are by now play-tested and fully functional. The fairness of allowing Presence to be nearly as helpful in dialogues, as Dominate & Dementation can be, was another achievement worth noting imo. The stability of the game is improved, too, as the console reports fewer problems the CPU has to handle.
In the one week within which I had private email exchange about VTMB with Dr. Spahl I found him a friendly, cultivated, and very well informed Patcher & Person taking great care to consider each decision & consequences.
As a Nosferatu Sympathizer I must note though: He, Toreador-Style, was NOT very eager to learn from oldie Vampire Redemption and make RATS, delicious Rats, pick-up bloodpacks for the Chosen. I still hope we get portable rats in patch 8.9 though.
I still think his patch belongs to the worlds best unofficial improvements of the game, too.