I am going to tell you a couple of secrets. The first is: really Aoskar, the god of portals presumably killed by the Lady of Pain really was one of the incarnations/variant of Vecna, a failed trick to rewritte the multiverse. A second secret is Azalin Rex, dark lord of Darkon was helped by Vecna to be a tool in a secret conspirancy to take revenge against the Dark Powers. The try to escape caused the fragmentation of the "Core" and the reboot of the demiplane of the dread.
The last things I read about Kas the Destroyer, the vampire is he was a dark-lord in Tovag, Ravenloft setting but later to become a vestige in Dragon Magazine #341 page 69 (I bought that magazine. I loved the article about lesser golems, the paper golem was super-cute).
* The Book of the Vile Darkness is like the Necronomicon (Lovecraft, and Evil Dead) or Darkhold from Marvel universe (let's remember Wanda, the MCU Scarlet Witch before and after reading that dammned book). It is perfect if you want to mix dark-fantasy and horror. Even it could be in a horror movie set in our reality, a wicked version of "isekai" fantasy. Entertaiment-One could produce some horror movie based in Hasbro/WotC's franchises.
* From the monsters from the 3rd Ed BoVD only I miss the kythons. If there is a future sourcebook I doubt we to see a previous UA article about crunch for players. The vashar and jerren are practically almost-identical variants of humans and halflings. Jerren shouldn't be more dangerous than the goblings or gnolls. Even kobolds with a right previous strategy about placed traps can be a worse nightmare.