So given that my laptop has a framerate of like, 3, I'm holding off on running any dungeons on my Demon Hunter until I get back from this trip tomorrow. Still, after trying out Vengeance during a demonic invasion, I'm actually feeling a bit more confident on taking the Demon Hunter for a run in the driver's seat.
We're at a weird place where half of all tank classes now wear Leather instead of Plate (and still no Mail classes, which is kind of odd.) Obviously Vengeance gives certain bonuses to armor and straight damage reduction to ensure that you can take hits like other tanks.
You have two major active mitigation abilities. The first is the more straightforward one - Demon Spikes. This has two charges on a reasonably short recharge and costs some of the Vengeance resource, Pain (which is generated by some abilities and I believe also by taking damage, making it similar to Rage, but not exactly.) Demon Spikes increases your parry chance and reduces damage you take.
The other defensive ability is Soul Cleaver, which hits targets in front of you in a cone and sucks in the various Lesser Soul Fragments you've generated (see below) to heal you. This one costs 30 to 60 Pain (Demon Hunters are really into randomness,) and of course needs to be "recharged" by getting those Lesser Soul Fragments out there, meaning you'll be using this less frequently.
Your main filler is Shear (an iconic Illidan ability that they obviously had to give to the class.) Shear will deal physical damage and then have a chance to shear off a lesser soul fragment from your enemy, dropping it nearby (like a more frequent Gift of the Ox for Monks.)
For AoE, you have a couple solid options.
Immolation Aura can't be kept up all the time, but it bursts for fire damage around you and then continues to pulse damage for a few seconds, generating Pain as well. Probably to be used on cool down.
Throw Glaive has no cool down or cost, and hits multiple targets, bouncing between them like Avenger's Shield for Paladins (so aim it for enemies on the side of a pack.)
Sigil of Flame is one of several Sigil spells that you place on the ground and let it go off after a couple seconds. This one just damages the targets, so it should probably be used on cool down.
Infernal Strike is similar to Heroic Leap, allowing you to hop a fair distance and deal fire damage to enemies where you land. It has two charges and should be great for dungeon groups if you don't need to pull them back (Throw Glaive and possibly Sigil of Silence would be what you'd want for that, I think.)
In terms of Defensive Cooldowns, you have Empower Wards - a 20-second CD that reduces magic damage taken. You also have Fiery Brand, which deals fire damage over time to the target and also reduces the damage they do to you for a few seconds (probably something you'll use rotationally, actually.)
And then of course, there's Metamorphosis, in which you transform into a big honking demon, increasing current and max health and generating Pain over time.
At the pull, you'll probably use either Throw Glaive to pull targets to you or throw down a Sigil of Flame and then Infernal Strike into the pack (at a distance, you can use Sigil of Silence to to make sure that casters come at you.)
Rotationally, you'll be using Shear to build up Pain and then periodically working in Demon Spikes, trying to keep them up as much as possible (and saving one charge in case there's an active-mitigation-specific ability you're watching out for.) You'll then use Soul Cleave if you've got Pain to spend but no Demon Spikes charges or if you need a quick heal.
You'll probably want to use Immolation Aura on cool down, though I don't think you'll need Throw Glaive after the pull, as IA and Soul Cleave should probably take care of your multi-target threat (also Sigil of Flame.) Shear should really be your filler ability.
I'll have a better sense of how tough the Vengeance spec is once I can play on my desktop, but at least when fighting demons in Westfall, I really didn't see my health falling very much except on the final boss, when lag made it impossible to get out of various fires.