We had a similar discussion a while ago, albeit about the Q400.
The Vmca and Vmcg actually reduce, not sure how much though, with the yaw damper disengaged. This happens because the rudder reacts quicker to your input as it's not trying to counteract any yaw you are trying to produce. This would seem to confirm Con's info assuming that the actual reduction in Vmc speeds was enough to bring Vmcg back below V1. Using Flap 15 also gives you slightly better lateral stability too.
I haven't come across this problem on the 400 before and I'm assuming because it's a bit loooonger than the old classics it doesn't have the same stability probs. Greater moment arms and all that.
Hope this is a help,
The Greater Chested One
PS I would be intersted to know the official reason.