Was there a specific reason you changed these around, or just design flavor? (Your version of the bite, by the way, should probably say "hits with both attacks from Twin Claws," or something.)
The thing about the bless vulnerability is that it was really a challenge for the players. And blessed melee attacks, while not exactly rare in D&D, were a lot less common than sources of radiant damage in 4E. I don't really like "trick" monsters in 4E, but I also don't want clerics and paladins to automatically just mow through these guys.
This, on the other hand, I like. I don't think I'd make it instant death, but definitely pile on some extra damage.
I also have some revisions to my original creature that I'll post a bit later. (Stats and defenses, and another power to make them more skirmisher-y.)
There are a number of reasons why I changed this around, while I am a huge fan of the recharge ability for monsters, I see it more as time spent building up to some power. For example; the time needed to create a magical effect, the time needed to build up that breath weapon, or the time needed to spot an opening for the attack. (Ok, so the bite can probably fit into that last description.)
I'll get back to this after I explain the claw attacks.
I would say roughly 90% of the non-minon monsters in the MM has
at least one other standard at-will attack besides their basic melee attack. A lot of times it is just a double basic attack, kind of what I was doing before.
You say you don't like trick monsters, how about a monster that runs around using their basic attack, basic attack, basic attack, then BAM! a bite that dazes and weakens the character (save ends both). If I was a player I would be like WTF?!
Speaking of dazed and weakened, in my research I have only found one monster that had a single attack that both dazed and weakened their foes. It was a solo monster and the attack power was an encounter power. Not even Orcus or any of the dragons have this amazing combination in a single attack.
According to the version of the bhut that I have a bite will numb the target. I think either dazed or weaken is a fine substitute for numbing, but not both.
So back to the recharge power. I was thinking something along the lines of the bhut having to set up the bite instead of waiting for it to recharge. I might have quickly tossed my thoughts on it instead of thinking about it. After thinking about it I think it should go like this:
Instead of having the twin claw attack the bhut has;
Maul (standard; at-will)
The bhut makes two claw attacks. If both claw attacks hit the same target, the bhut makes a secondary attack against the target. Secondary Attack: +11 vs. AC; the target is grabbed (until escape).
Cold Bite (standard; at-will)
The bhut deals to a grabbed target 2d6 + 3 cold damage, and target is weakened (save ends, no attack roll required) this ends the effect of the grab.
Also in the version I have, a blessed weapon will kill a bhut, automatically. Now maybe a vulnerability of 20 is too much, but I can't see 10 being so.
Maybe a change, a critical hit from a weapon with the radiant keyword brings a bhuts hit points to 0.