In many light twins the addition of flaps will increase Vmc. This is due to the accelerated slipstream on the side of the operating engine blowing over the flap which can effectively be thought of as an aileron. Increased airflow over that flap increases lift on that wing and increases the rolling moment towards the dead engine thus increasing Vmc. Vmc demos on a Seneca for example are (or should) be performed with 25 degrees of flap (and gear up). This is the 'worst case' scenario for that plane and is the configuration on which the published Vmc is based.