iserith
Magic Wordsmith
Though I agree with you, I've always felt as though changing a monster's vulnerabilities in an attempt to catch a metagaming player in a trap is almost a double sin.
I almost always prefer assuming that the characters know, assuming that they don't know but they're fed the exposition about the vulnerabilities at the same time that they discover what the creature is, or just avoiding the entire situation by not putting a vampire, werewolf, or troll in the encounter.
I don't see it as a sin, just another way to increase difficulty. I frankly don't care about "metagaming," so I've no issue with throwing a vampire into an adventure and watching players gather up stakes and garlic without additional prompting. But it's a wonder why those who do care frequently (at least in my experience) resist changing monster vulnerabilities or the like to remove the possibility of it occuring.
I also always telegraph a monster's strengths and weaknesses so that observant players have an opportunity to create an edge for themselves. I don't care for gotchas.