LostSoul
Adventurer
The problem isn't with the idea - using the vulnerability is a good idea. The problem is the execution.
Using a standard action, even with an attack roll, just doesn't make sense. That's way too easy. Why can't I grab a bow as easily? A player should NOT choose to play an archer if you don't want close quarters to be problematic.
If someone hasn't fastened their hood well, and if they aren't armed - then sure, a standard action is reasonable. Otherwise, I'd say this is on the order of disarming - that's being rather optimistic, IRL this would definitely be harder - and disarm is not something trivially possible in 4e.
So yes. You're the DM. Be creative! Don't just unbalance the game by introducing inconsistent combat actions, make something up that players and other creatures can't copy willy nilly.
A couple points:
1. What P1NPACK suggested requires two standard actions, with attack rolls for each and the horrible Grab action, neither of which actually causes damage. The ongoing damage can be countered easily ("Take my cloak!").
2. Yes, you should be able to grab someone's bow from them.
3. Disarming is fine, even if you're not a 17th level Fighter; you just have to keep aware of the action economy (unless you're a 17th level Fighter, a disarm should not deal damage).
4. None of this even comes close to unbalancing the game. Personally, I'd say the opposite: it makes it more balanced.
Why wouldn't this be v. Reflex?
This is what I use to determine if an attack is vs Ref or Fort:
You try to push, overpower, or manhandle someone: Fortitude
You try to touch or tag someone: Reflex
Ripping the cloak away seems to fit it the "manhandle" definition, so I personally would go with that.