Why is the Vial so rare? Because getting it requires that players max out not only their Alchemy, but then also Archaeology, since the formula to produce the vial is only found in Tol'Vir digsites which: 1) Do not appear at all until 450 skill, and 2) Even once you're at 450 skill, they're at best a 15% chance of appearing.
Even then, the formula itself is not assembled directly, but is a rare drop from the also not terribly common Canopic Jar artifact.
So, in short, the Vial pattern is rare because it requires two maxed professions, and an incredibly long grind to locate.
That said, even if someone on your server has the pattern, the odds of it showing up on the Auction House will still be somewhat low because as Raven Dreamer points out, the materials required to create one are incredibly expensive, and largely bought from a Vendor. Thus, even for those with the pattern, they may not be interested in or capable of expending the capital required to produce one just to have it listed on the AH, preferring instead to produce only on commission.
My suggestion? Get the materials together yourself, and then look for someone willing to craft it for a commission, either through your realms forums, or simply in trade chat.
Right now arms seems to be the beefiest of the three specs (a year ago I'd have said prot). They have a decent ability to heal themselves through [Field Dressing] and [Second Wind]. If you combine that with Victory Rush which heals 30%(?) max health after a killing blow, they can be pretty tough to kill when they can kill really well.
Mortal Strike is the main ingredient that you're looking for here. It reduces the healing received for the victim and there are a few talent points you can spend so that it does more damage like [War Academy], [Wrecking Crew] & [Lambs to the Slaughter]. Plus it can be glyphed for another 10% dmg.
Another ability that really helps with dps is Overpower (does good dmg and happens a lot with [Taste for Blood]). Plus it can also be glyphed for another 10% dmg.
Those two, plus bladestorm, plus bleeds, plus debuffs which increase dmg taken... these guys are tough and when played well will make small work of decent toons.
Hope that helps!
Best Answer
Based on the Wowhead discussion of the NPC there isn't a definitive answer, but it seems that you have to be on one of the Outland breadcrumb quests, for example Warchief's Command: Outland.