I have no specific background with the Airbus and am not able to offer any thoughts at the present .. but I will have a head scratch and see what I can come up with ... perhaps a revisit to the AC might give me a clue.
However, unless OEM data indicates explicitly that Vmca was done wings level, you probably would be better off presuming that the figure is for 5 deg into the operative engine - too much of a penalty otherwise as a general rule.