I tend to agree with a lot of people here, there is a lot of blame to go around.

As a raid leader, unless something is so much on farm you can sleep through it (like doing Utgarde Keep at the end of wrath), vent was always very important to me. So much so, that I would do a Vent Check. This is a really simple tool, and here's how it goes:

"I'm going to hit ready check, and if you can hear me, I want you to hit NO you ARE ready."

Usually one or two people hit yes that are on vent, and they will then say on vent "Oh heh yeah i'm here i just spazzed out." But the 1 or 2 people who did hit yes, and didn't respond, that way I would know they aren't on vent.

Primarily, I find fault with the raid leader here. Secondarily, VT really reacted badly. I would say he reacted about as bad as someone can act.

The bottom line is that this is a miscommunication. Miscommunications can be extremely problematic, and in certain occupations can even be harmful or deadly with drastic consequences. (The BP oil spill is a prime example of extreme miscommunication error.)

What VT should do, if he wants to stay with this group, is own up to his mistake, tell the guild and RL he is very sorry for acting out, and will in the future work to assure he knows the fights better and has his vent cleared up. Any less and he should just cut his losses and head somewhere else.