Yes. I would. Fairness? Fair means the PCs can do the same thing to monsters. That's fair. If your PCs want to drag a vampire out into the street and remove its cloak for a nice suntan, are you going to tell them, "No, that's not fair!"
Come on, dude. That's bogus.
Yes, it's not fair. It wouldn't have been in a killer 1e game, and it's not in 4e either. It has nothing to do with edition, and everything to do with consistency.
If you institute a novel game mechanic specifically aimed at using a PC weakness, with the in-game explanation that this happens by means of affecting the PC's use of his equipment, you should be consistent. Anything else
is unfair.
What does that consistency mean?
- It means that other equipment by other creatures (including other PC's) should be similarly liable to be affected. The rule should not be specifically designed for the vampire, but be general. If you've never snatched someones headband and suddenly introduce mechanics to do so when the vampire comes along, that's certainly looks unreasonable. I know if I were a player in such a campaign, I'd take it less seriously right there - I don't like opportunistic consistency.
- It means that similar effects for creatures should be possible, where reasonable. However any and all creatures in the MM aren't designed like PC's; which part of their power is due to which aspect of their physiology/items/situation/etc. isn't defined. If you're going to do this, don't make the player's go on a fishing expidition. Tell them of vulnerable parts of monsters, blind angles, equipment etc. that could be similarly exploited. If you aren't, you're being inconsistent - you're doing it to PC's not because it makes sense, but because that happens to be easier in a metagame fashion.
- It means you should be reasonable in your use of the new rule and given how weak this particular vulnerability is, that means that it's probably more relevant to use your equipment-fiddling power in another fashion.
- When the vampire (inevitably) actually ties down his hood, you should let him. Whee, all this effort... for naught.
If you come up with an equipment-fiddly power and then go "well, I'll just use it on the vampire, and since you don't have this kind of precise desciptions of monsters, good luck using it yourself" then you're being inconsistent. If you do this without warning the player before he brings a vampire to your table, you're screwing your player. Not in a major fashion, but I don't think it's fair, and I wouldn't do it.
This has
nothing to do with using the vulnerability, and everything to do with the fact that the proposed solution just doesn't make sense. If it
were possible and that easy to move other creature's equipment, using a minor vulnerability that occurs at the end of the turn would
not be the most attractive way of using it. And it's unreasonable because monsters are likely to be largely immune not because of any particularly logical reason, but simply because their descriptions aren't as detailed, and thus it's harder to think of something comparable.
Not to mention the fact that all of this has absolutely no support in the game. Now, there's nothing wrong with houserules, but some of them work better than others. 4e's strength is it's intrical tactical ruleset. This is like trying to tack a simulationist approach that doesn't mesh with
any existing rules. Indeed, 4e is probably one of the worst possible rulesets to do this in; character builds+tactics are quite complex, and the rules concerning PC's bear no resemblance to those for monsters. You'll need to do the whole thing from scratch. That, or be inconsistent, and apply it only to the vampire or only to PC's for no particularly logical reason.
It might surprise you, but I don't think that's at all bogus. Particularly since all this "general solution" approach is totally unnecessary. Make it a unique occasion - indeed, DM fiat - and just go with that plothook power. No worries about other uses, nor odd interactions with game balance, nor the need to explain away inconsistencies.
Then there's the more abstract problem I have with this approach. I don't mind simulationist gaming. But 4e is anything but. It's got a complicated, balanced, shamelessly gamist combat style. I feel that adding complexity to that with rules intended to simulate some kind of in-game action just doesn't work well. There are
tons of 4e combat rules that are very weird or outright nonsensical from a simulationist perspective - which is of course entirely not the point. Why exactly is this one worth fixing? Why not marking? Or prone gelatinous cubes? Or immunity to backstabbing? Or that solo's miraculously have four times the hitpoints and move twice as fast? Or whatever?
So you want to make the game better - but then you
don't need all this detailed description and simulationist power-design. Just have a creature that happens to be particularly nimble and tricky and let it do what no other creature can - play with the PC's equipment while they're wearing it. Including the vampire's cloak. No need to be consistent since it's
not a general solution, and there won't be anything to compare it to with which is should be consistent in the first place - it's story logic, not general the-physics-of-throwing-back-someone's-cloak logic.