"Vmca" as a published limitation speed applies only to multi-engine aircraft - the lowest speed at which the rudder (or other flight controls) can maintain straight and level flight with one engine failed (asymmetric thrust). Slower than that speed, and the rudder will be at full deflection, and yet still not providing enough yaw force to maintain straight and level flight -
whether the wing is stalled or not.
Since there is no such thing as "Vmca" for a single-engine aircraft, even though a SE aircraft will have a stall speed - obviously the two mean different things.
I can see where some confusion sets in, however, since a basic maneuver in (single-engine) flight training is called "flight at minimum controllable airspeed" - maintaining altitude and making gentle turns while on the edge of the stall. But without a failed engine in the equation.
Maneuvering at Minimum Controllable Airspeed
But "Vmca" - for a multi-engine aircraft* - means something different than paddling a single-engine Cessna or Piper around the practice area with the stall buzzer screeching. Despite the identical wording.
*always excepting the "push-me-pull-yous" like the 337.