If there's a nerf on Vengeance in 5.2, i would be really interested in knowing more about it. Did you have time to do some tests ? An important part of a Brewmaster's active mitigation comes from Attack Power, so it's essential to know exactly how Vengeance is or isn't modified. If you have any information, i'll take them.
So far, i've noticed that a good way of making tests about Vengeance was to find a lvl 90 mob with no special attacks, only melee autoattacks (they're not that common ; flying mantids around the Gate of the Setting Sun can do the job), let them hit you for a long while and heal yourself with Healing Spheres, without using Keg Smash at the pull so that the mob doesn't have the Weakened Blows effect (it's not a problem if it has the debuff, but you'll not Keg Smash the mob during the test, otherwise the mob will die and you won't have spent enough time being hit for the test to be relevant ; so you should take into account the Weakened Blows debuff only for the first hits, which is unnecessarily annoying).
What you actually need to know is a precise value of the average damage of the mob, its swing timer, and the Vengeance this damage gives you. But i would understand if you don't go through these steps on the PTR, since such tests can take some time and are not particularly interesting.
And actually, as long as Blizzard doesn't change the way Vengeance ramps up when you're autoattacked, just knowing the value of the first unmitigated damage taken, the swing timer of the mob and the Vengeance given by that first hit is enough. Only... the low Vengeance you observe can come from a modification of the way Vengeance ramps.
Also, dodging/parrying the first hit will give you Vengeance depending on the average unmitigated damage of the mob. But this is useful to know the average damage only if you also know the way Vengeance ramps up... In 5.1 you should initially get Vengeance from the first unmitigated hit equal to :
0.5 * 0.36 * (damage_of_the_hit / swing_timer)
if i'm not mistaken