I've had reverse problems. Or rather, the same problem on both sides.
I tank with a Prot Warrior. After either one of us does a swap (After a period long enough to let vengeance drop off) the one who's no longer tanking the boss must stop attacking. This is something I've only noticed in ToT, we never had the issues in 5.0.
I've been helping the issue by swapping into frost presence if the damage is needed and swapping back once his vengeance is up a bit unless it's vital I remain in blood presence for the entire duration. If that is the case, I'll simply stop attacking (Much).
I will mention, that I am capped at 7.5% for both melee hit and exp (bit over on hit atm, but that's something I know about, didn't have time to really get into shit during a 10 minute break during a raid ). My Threat gen still seems rather low in some cases where I'm struggling to remain above our DPS including our Frost DK who's rockin' 523 ilvl gear.