The extension of flap can affect Vmca if the aircraft has a blown wing. I can only speak for the P-3, which has a lower Vmca when you lower flap, but this is negligible and not considered practically. What happens is that the flap behind the operating engine generates more drag than the flap behind the failed engine. The higher drag on the operating engine side creates a yawing moment that opposes the moment due to the asymmetric thrust and thus the net yawing moment is reduced. This in turn means a reduced Vmca. Hope this is what you were looking for.
