Regarding tank healing needed, Vezax is pretty much a question of tank gearing and fight length. Vezax hits hard and fast as you noticed. For 10 man, let your tank maximize his gear for mitigation instead of HP for this fight. A few percent additional avoidance is very noticeable. We run Vezax with a Warrior wearing 10 man gear exclusively and no regular tank deaths whatsoever, so going away from HP towards mitigation should work for you, too.

Also note that Vezax seems to have parry haste. I'm not sure this has been verified to be true, but have a look your tank gibs for hits. Have a look at your melee positioning and expertise rating on the part of your tank.

Armor potions (one immediately before pull, second within the fight) also help.

Is this first enrage on a strict one minute timer? I usually save a bit of runic power the first minute or so because I'm not sure when the first enrage will come. But if I pop IBF on the way in will it be up by the time the first enrage comes around? I tend to take a lot of damage the first few seconds before debuffs get up and this would be a good way of mitigating some of it if I know my IBF will be back up by the first enrage.

A bit late reply, but this is where your problem lies.

Make your disc priest do nearly nothing in p1 but throw shields on the Mt (from shadow crash) and the occasional penance when a ranged dps messes up alot and is at -15k hp.
Make sure your pala keeps SS up on the tank and just heals with flash (the holy/deep prot build with SA and the spellpower and crit talents is sick on vezax) once your tank drops under 75% or so.
The pala I did it with yesterday had ~32k mana buffed (double int trinkets, no plea) and 7% overheal in the end. All he casted was sacred shield (from shadow crash) and flash of light. No way a disc priest can come close to that mana efficiency.

When p2 is about to start (priest should be on 90% mana here, if done correctly noone takes dmg in p2 that JoL can't heal), make sure the entire raid is shielded, and the priest keeps doing this. In between, inner focus divine hymn and some group healing. Pop Disac at 40% or so.

I must say that with your setup, adding a ret pala will help alot. Strong jol = no healing needed on dps when they do mess up once on shadow crash/mark, extra SS and best of all, extra DiSac.

If you pop IBF the first 3-5 seconds into the fight, you will have it back up in time for the first surge. Any later than that and you risk not having it up.

Runing in and out the crashes and back to melee range doesnt seem worth the hassle to me.
With meleeing all the time there will be lots of procs and casting lifebloom of a proc will result in a mana gain.

We had a healersetup with 2 Resto druids and a disc Priest and it seemed like we could hold up the tank for most of Phase 1 with cc procs and occasional disc shielding/penance. We healers did not bother with going into the saronite clouds. The risk of spreading marks , having to run and having bad crashes seemed just not worth it.
What did help our Priest a lot was calling out when we were going to Regrowth so he could save Mana instead of worrying of the Tank being at 50%.

Overall the fight seems to be very well tuned fight at least in 10 man.

What I am interessted in is how big the difference in the 25 man version of a Blood DK compared to other Tank classes like for example a feral Druid is?
Invastigating combat logs for incoming damage on different Tanks should be quite significant?

No idea about a DK but I can safely state that a warrior tank is much more fragile than a druid. And yes, the difference is significant; one tank lives through surge+animus, other tank dies randomly in the fight; with the same 3 healers.

Another point is, the fight becomes much, much harder if you try to get the mana buff. I'd go as much to say that hard mode Vezax is in fact easier than the normal mode. Just pop a random cloud if you don't want the hard mode, or well, kill him before Animus or something, but running across the map in order to fill mana is actually more costly than simply sitting at the same concentrated ranged spot, unless your entire dps is in melee.

About 10man hard mode, we tried both 2 and 3 healers and 3 healers felt way more stable while damage was still sufficient (25man gear). Our setup:

Feral druid (MT, not kiting)
Prot warrior (Animus, debuffs on boss, interrupting)
Disc priest (shielding from shadow crash)
Resto shaman (saving mana for p2)
Holy paladin (healing in p1)
Rogue
Warrior
Hunter
Mage
Ele shaman

We had Vezax at 30% when p2 came, our hunter died when animus was at 10%, but we still managed to finish Vezax (but it was very very close). In my opinion optimal setup for non-kiting tactics is 3 healers, 1 tank, however it may be gear dependent.

Why is it a tremendous boon? They don't need the mana. Animus is what makes hard mode hard, not mana pools themselves.

Running after clouds causes a chaos. You might say that this experience is merely a result of bad players/strat and I have nothing to say against that. All I know is a hard mode try is a very stable one; and clouds add one more thing to worry about into the fight, totally screw up positioning, reduce dps by scattering shadow crushes, intentionally damage people only to get oneshotted by a shadow crush, etc.

Now, hard mode maybe isn't easier, but managing those clouds is one hell of a burden that it makes hard mode look not so hard.

I have no clue what you are doing to kill clouds. We have someone dps one dead near the melee -- where the healers are normally. Healers step a few steps out and get mana. Done. Not hard, not a lot of management.

Having a deathgrip for clouds removes even the random factor of there being none near melee when the healers want it.

In other news, I'm curious as to whether anyone has solo-healed hard Vezax (until Animus), in 25 or 10, with anything other than a disc priest. (I don't even see how 25-Vezax is solo healable without a lot of luck, no matter the spec.) I can certainly do a bit with clearcasted HoTs as a druid, but that's not always going to keep him up all by itself, and if I get really bad luck with CC procs, I won't last with my current tactic (at least one HoT up at all times; ready to swiftmend/nourish any spikes; autoattack otherwise).

(I neglected to add: we have a paladin solo 10, with me supplementing with CCed HoTs only, so I know that's doable.)

Honestly, I think I agree with Plea up until the point the Animus spawns. Ignoring clouds makes for a much, much more stable fight, at least in 10 mans (havn't gotten to Vezax in 25's yet). 100% of our wipes learning Vezax came from one of two things: Searing Flames not getting interrupted, or healers not healing. The healers weren't healing either because they were out of range because the clouds got killed in bad spots, or because they were running to a cloud. I'm generally our only DK in 10's, so there's no death grip for the clouds, which means healers have to move for the clouds.

We don't wipe on his normal mode any more, but ignoring the clouds is MUCH more controlled. I think if I were recommending how to learn Vezax, I'd recommend killing one cloud to start the fight (to prevent the Animus from spawning), then just ignoring them for the rest of the fight. Not only does that start training everyone for Hard Mode, it removes several points of failure.