Vmca is most critical; at MINIMUM weight. This is because the amount of bank into the live engine is limited - by regulation - at 5 degrees.
For a given weight, the sideslip to balance 5 deg bank can be determined; the heavier the aircraft, the more sideslip you need to balance 5 deg bank. Since that sideslip HELPS control the asymetric engine, it follows that the critical case is where the sideslip is minimized, i.e. the lowest weight. Therefore Vmca is highest at the minimum weight.