Vengeance don't get worse with more avoidance, or better doing /sit. Vengeance is a 2% of the melee damage you would have received with 0 armor, no avoidance, no block, no % for stances, no nothing. Parrying 10 attacks on a row gives the same vengeance that getting naked and giving your back to the boss.
Magic damage was... 5%?
Anyways, vengeance will keep scaling with content, and better gear won't make it worse. NOR better.
On the other hand, dps should getter better gear instead of complaining of tank dps, which is completely undependant of our gear (barring hit/exp), only of the encounter. I recommend arenas, free 383 gear.