VIT actually does save bad in my experience, in the case of making sure I have enough time to get DoTs up. Even most bad tanks will hit at least one cooldown for a large pull, and with the extra health, I usually get the extra GCD or two that I need. In part it's a UI issue--I don't know many healers that sit and tabulate health totals mid-encounter (nobody's a human calculator after all). They're normally relying on the fluctuating health bar to gauge the need for heals, which results in more frequent stance-dancing which is a flat DPS loss.
You're also using a lower number for the HP which makes your argument appear stronger. On my tank (a measly i173), dropping my Fending accessories costs 3444 HP (3557 with party bonus), which is about 1.5x your 2k HP estimate. That difference between all Fending and all Slaying is more pronounced at higher gear levels, naturally.
This is across a varying number of item levels, of course. Honestly, as long as I see the tank has around 16,000-17,000 hp (for DRK or PLD at any rate), they're as good as a VIT tank to me regardless of whether they're wearing Slaying or not in the Expert dungeons, as that's plenty of HP to do the majority of the pulls fairly safely in my experience. It's the sub-15k tanks that tend to give me pause.
But in any situation where I don't know the tank (i.e., most DF runs), I'm always going to be initially happier zoning into a Fending tank, as it's the "safer" option.
