You can gain a stance through the Martial Stance feat, if you first acquire a maneuver from the same discipline (usually through the Martial Study feat or a martial adept class).
Vorpal, in 3.5e, only works on an attack roll of natural 20, followed by a successful threat confirmation roll. As a result, it doesn't matter what weapon it's applied to, it'll always be the same effectiveness.
3.5e design just has some wierd stigma against high threat ranges. Back in 3.0 when the bladed gauntlet was messed up in Sword & Fist, there was a possibility of it being problematic (fighter 6/weapon master 7 with improved critical and a keen +1 vorpal bladed gauntlet? instant death to most monsters on a natural attack roll of 7-20. Before getting vorpal? Still a crit on 7-20, and weapon masters can boost their crit damage multiplier a few times per day. Before weapon master 7? Still a crit on 9-20.).
But aside from that misprint (Arms & Equipment Guide fixed it, or rather over-fixed it, as 19-20 threat range, making it a sucky exotic weapon that was no better than a short sword), it wasn't a big problem. While the average rapier-wielding fighter/duelist might have a threat range of 12-20 with improved critical and a +1 keen rapier, it wasn't very significant damage; that same fellow might be dealing 2d6+5 damage normally (less than the average greatsword-wielder, at the cost of more feats and class features), or 3d6+10 on critical hits (duelists in 3.0 had Precise Strike, but it didn't multiply on crits since it was bonus damage dice).
All in all it roughly evened out, since they'd get kind of frequent critical hits for slightly more damage than Joe Greatswordsman (though JG would do more damage on his own, less-frequent crits), but at the cost of more feats and class features and with much less power against undead, constructs, oozes, and whatnot (only 1d6+5 damage against them and no crits, and piercing tends to be less useful against some of those critters).
Anyway, in 3.5 they tried to cut down on critical hits, and made most threat-range increases worded so that they wouldn't stack with anything else of the sort.
Your stance is more powerful than Improved Critical in either version of the game, anyhow, and while I don't think it's as awesomely powerful as some people do (I'm more of a realist; seriously, how often are you going to wield several different weapons and also manage to score more critical hits with them, just because you have some ability that slightly boosts all your threat ranges? It's more like two or three IC feats in potency, since that's how many weapons you might realistically use it with, unless you limit it to just the Discipline's own weapons, in which case it's not even that powerful), I still consider it worth a 6th, 7th, or 8th level stance in cost/effectiveness.
I'm kinda hesitant about making it higher than 6th, though; the 8th-level Stone Dragon stance (which is available to all martial adepts, as long as they take a few SD maneuvers earlier) gives immunity to critical hits as long as the character doesn't move much and stays on the ground. I'd have to estimate a +2 threat range stance to be no more than 7th-level, tops, if it worked for all kinds of weapons. 6th is probably good otherwise, and heck, after thinking about it this much, I might even be crazy enough to consider it 5th-level.