Vengeance does feel awkward, and even illogical right now.
Something that scales with incoming damage that is ideally supposed to scale with the TPS of the people below you, will never even out in any form of fair balance when it comes to skill vs not-making-it-too-hard mentality that blizzards seems to run with these days.
Average tank TPS should be relatively equal (if done adequately) to the average DPSes TPS on the same gear level. Having a flat-out static threat multiplier on, for example, Defensive Stance does accomplish this without letting tank dps grow too high.
On paper, making tanking gear+talents x% lower on damage-output compared to pure dps gear+talents, then multiplying threat by that same x% should create all the needed balance.
This especially works well for warriors since 90% of their threat seems to come from pure damage now. A mechanic like Vengeance just makes that balance bounce all over the place.
Higher geared DPS should need to hold back to prevent overaggro.
Lower geared DPS should be able to go all out.
Equally geared DPS should live for the chase.
Why create complications with things like Vengeance in the first place? Tank gear is as much part of progression as anything, and should provide the necessary scaling; not some silly mechanic.
On another note, innitial aggro should be tricky in my opinion. And 5mans have always required more effort from a tank than any average raiding encounter has. (No I'm not talking about post TBC and Pre 4.0.1 faceroll AoE fest)