So yeah, I've tested this almost 20 times and have logged out for at least half of those.
Since this patch, switching from my OT (Arms) to my MT (Protection) completely renders Vengeance invalid. I don't get the buff, I don't gain any attack power -- nothing.
I first noticed it when doing a mount run in Stonecore. Usually I just drag everything I don't need to kill to Slabhide to raise AP and kill it quicker (since second wind will keep me alive). After a bit I noticed my crits were suspiciously low and sure enough no Vengeance.
Submitting a bug report atm and posting on the bug forums but figured I'd post here as well in case anyone's noticed this since the patch today.
(Yes, I tested with zero addons as well -- it's flat out broken when switching specs.)
So yeah, after trying to fix it after both posts I'm now a tank with no Vengeance. I'm pretty sure I could hold hate since the thresholds are high enough on base values but that's a *LOT* of lost damage during dungeon runs since I'm trying to gear up.
I fail to see what your point is. Damage is damage is damage. I hardly think that a 50K nature hit (I let the quakes hit me just to make sure a large enough hit got through to ensure I'd see *SOMETHING*) is weak enough to bypass, not to mention I normally have had almost 35K from the mobs when doing this yesterday.
That said, yes, I did swap and test out while trying to do dailies as Protection. That lasted all of 5 minutes when it took bloody fucking forever to kill three mobs.
---------- Post added 2012-10-10 at 10:51 PM ----------
I also double checked to make sure that I didn't herp-a-derp and try to tank in Battle Stance. Nope, still in Defensive so that's not it.