Captain Mo is correct, it is an increase in weight that will decrease Vmc, not the opposite. You must seperate performance and control. Vmc is about directional control, nothing else. When an engine is brought to idle with the other running at take off power, you will see a very strong yawing tendancey, yes with roll as well. The natural instinct is to apply opposite rudder. However, when you are slow, you must apply more rudder to get the desired effect. When you run out of rudder travel and lose directional control, that's Vmc. The red line is Vmc under specific conditions. It is meant to be used as a reference.

Anyway, back to your question about weight. When you bank the aircraft, you create a horizontal component of lift. The reason we lift the dead engine is to allow this horizontal component of lift to assist us in counteracting the yaw which was created by the good engine. When we increase weight, we increase total weight, which creates more horizontal lift. The more horizontal lift, the more lift counteracts yaw. Will you maintain altitude, it doesnt matter. We are talking about Vmc, otherwise you could crank it over to 45 degrees and you'll get Vmc very low. This isnt very safe now is it. But the increased weight at 45 degrees, you may even rudder in the same direction as the dead engine. But this is all about Vmc, not performance; or safety in this case.

Back to weight, this is also not to say that you should fly around at max weight. It will dampen your performance. How often will you be flying at Vmc, hopefully when you are on the runway and its a split second as the needle passes over it. But if you want to minimize Vmc, then by all means, load that bad boy up.

Another intresting topic, did you know that having the gear down will decrease Vmc as well? Now thats counter-intutive.