To understand this its important you´re familiar with the term
P-Factor...which is the asymmetrical thrust caused by the descending right blade of the propeller creating more thurst than the left one (asymmetry gets greater w angle of attack)
Imagine a situation where the RIGHT engine is failed....The left engine is producing thrust, and the RIGHT blade of the LEFT engine (closer to the long. axis) is producing more thrust than the LEFT blade.
Now if the LEFT (critical) engine fails...then the right engine will be producing thrust and (at high AOAs) the RIGHT blade (the one farther away from the long axis) of the RIGHT engine will be producing the greatest amount of thrust...
Now...the more flaps you have, the less angle of attack you will need to create the same amount of lift, if the angle of attack decreases, then P-Factor decreases, and the yawing moment decreases too... that´s why the amount of airflow the rudder will need to counteract the yawing moment will be less.
The opposite takes place in airplanes with counterotating engines. The more flaps you have the higher VMCA will be...draw you a pic and think about it.
Cheers