The use of flap lowers Vmc beacuse your AOA reduces hence a bigger fin area is exposed in the stream flow.
The rudder is the primary control related to Vmc, so assumed the CG remains the same, so the arm, a larger rudder exposure in the airflow ensures a positive controllability also at some knots below the same situation with a lower flap setting.
Generally speaking, many aircraft models have more selectable flap positions within the flap take off range, so the use of a higher one reduces the Vmca,althought it affects other factors as the climb gradient and so on.