I see the designers really like the Last of Us, but didn't want the infected to be zombies. I like all of the Sporeborn critters.
I like how scary the tone is for a lot of these critters, although some of the humanoid write-ups (particularly the orcs) are close to bio-essentialism again.
The ghoul is, as expected, lightly leveled up and made scarier. I hope you can sneak in a few more ghoul variants, although I know you don't want to cannibalize your other monster books too badly.
Good modron replacements.
The void stuff is
creepy.
The variants in general are so good, I'd love to see them either replace the regular versions or get a standalone write-up. After this many years of playing D&D, I'd be happy if all trolls were stoneskin trolls, who were damaged by radiant damage and turned to stone in the sun, for instance.
Not sure how I feel about stats being replaced just by stat bonuses. For the people just using Tales of the Valiant -- which is presumably what you're hoping will happen,
@Marc Radle -- it's fine, but for what I suspect is the more typical use case, in which people will mix and match books, this hurts compatibility. I don't want to have to look at a cheat sheet for stat bonuses when I'm using the Monster Vault in the middle of combat.
In any case, extremely well done. You've made a great case for the Monster Vault as a product.
