In that situation, the one you stated about enough HP + mitigation, and more HP + unmitigated) I would agree and if the tank pulls off the mitigation correctly, then that is considered skill and experience. That is the case of where everything would fall in place perfectly.
The assuming factor in your situation is the state of the healer. There are a lot of factors in determining how a good situation would turn very bad and most of it stem from your healer. Exaggerating, if the healer suddenly has a problem with his hardware, or a situation in which he cannot heal (such as he died), the excess VIT will decrease the chances of a wipe. The other healer can possibly keep you up at enough health to raise the dead healer or replace him for a short while. This is the situation which I had brought up.
Any tank busters in the game would not reduce the tank's HP to zero, merely to a critical point. In having a certain amount of VIT, that critical point can be avoided, therefore decreasing the margin of error. Again, this is up to the healer. If he is prepared to heal you up, that's good. If he does not? Then your extra VIT kicks in and you may survive long enough until the heal comes back again.
Then, therefore by stacking VIT in excess and he knows he does not fully well know certain mechanic (I assume mechanics such as big hits, and so on) is a reasonable choice and does not perpetuate that the tank is bad. Merely, inexperienced in said content. Once he has known how to handle game mechanics and gearing choices, I assume he would want to go STR. Again, this is a choice for tanks.
However, stacking VIT in excess does not REALLY mean he doesn't fully understand the mechanics of the game. He may have chosen so because it was his preferences to have a certain HP threshold to go certain contents.
What I'm getting to is, excess VIT doesn't really mean the tank is bad. It means he has excess VIT, that's all. These judgements that excess VIT is a bad thing for tanks is, in my opinion, not true. It merely says that the tank has more HP, that's all.
In regards to comments that the excess VIT can be exchanged for something better, such as STR, I would say that is also a reasonable choice. Provided he is confident in his skills to survive any kinds of mistakes or slip ups from his teammates.
I know that I sound like I support the notion that we need more VIT or pro-Vit build but I merely want to convene that having a little bit of excess VIT (or more) does have it's own merit and cannot be shrugged off as a bad choice.