Yeah? Why not? You tell me. I'd certainly allow this. lol If I can do this in real life, why not in a fantasy game?
Right, and that's perfectly reasonable. However, I suspect that most DM's don't actually do this. Although PC's (and presumably NPC's) are heavily dependant on gear, they never rip it off or deprive them of their use. Worse, they never describe in sufficient detail similar weak spots in monsters. So in-game it may make sense to win a combat by "attacking" the other guys gear or specific weak spots, and it
theory DM's are OK with it, but in
practice they aren't actually doing this: not to the players, and certainly not to the monsters (which would be much more work because 4e's MM doesn't describe such weak spots).
Presumably there's an in-game
reason for the various strength's and weaknesses of monsters. Just like you'd be exploiting the
reason that the PC vampire can survive in sunlight, it makes sense for PC's to exploit the
reason the monsters have those strengths and weaknesses. And it makes sense for other monsters to exploit the
reason for other PC strengths and weaknesses. Ideally, you'd have been "attacking" gear all along, and have notes that describe for each monster you've ever used in the game the means by which they fight in similar detail to PC's.
So if you pull back the hood of the vampire once; that's dramatic license. If you're doing as a general strategy (e.g. any monster might do this, it's a viable combat action), then you've got to ask yourself what about all those other
reasons - why am I picking on this one?
And then the point is that this just doesn't work well in 4e. It doesn't lend itself to simulating in-game effects very well; it doesn't describe why monsters work as they do; they just do. The whole point is that making monsters is easy and fun; you don't want to explain it in too much detail: you look at the results, not the internals.
Or, even better, how about wrestling a dragon down by its horns and ripping them from its head? Yeah. I'd allow that too. That's badass. But, there's no "power" for it.
And this is a perfect example of something that's badass
once, but you wouldn't want to make this a general rule. It happened to work against one particular dragon in one particular situation - it's not generally valid. You're not going to let them systematically apply this action all the time -
regardless of whether it makes sense at first glance. You refluff the story to match the rules, and then use dramatic license where appropriate.
Same thing with the vampire. Use the weakness once, dramatically, and make it cool. But don't turn it into a general mechanic. That's almost certainly inconsistent with how you treat other PC's and certainly inconsistent with how the PC's can treat monsters.
DMG42 doesn't really say anything about this at all, but the spirit of the rule concerns
unusual actions. That's why I was suggesting using some kind of blink thief. It's sufficiently unusual that no-one's going to worry about this being a constant threat, and you can do imaginative things without messing up game balance - because once he's gone, the rulings don't matter any more. The mechanical suggestions you made (using grab, etc.) are certainly better worked out; I'd say; go for it - just use an explanation that makes it intuitively obvious to the player's that you're introducing a new challenge, not a new rule.
It's not about
whether to use the vampire's weakness - you
should use the weakness, because players' choices should matter - why play a vampire if it never matters? I'm critical not of the use of the weakness, but of the suggested means. You don't want a systematic new combat mechanic, you want a novel situation calling for creative solutions. So don't overdo it, and when you do, choose an in-game explanation such that it
makes sense that it won't happen every combat. That way, you can also make the effect more powerful (i.e. dramatic), since you don't need to worry too much about balance.