Funnily enough, Vmc goes down with gears out (as they act as rudders and help stabilize).
That might be true if the gear was at the rear of the aircraft, like the rudder is.
However, the main gear is generally very close to the centre of gravity, and will have almost no effect on the stability of the aircraft - at least, not compared to the nose wheel, which is generally well forward of the centre of gravity. Because it is forward of the centre of gravity, it will actually
de-stabilise the aircraft in yaw, and
increase Vmc.
Counteracting that is the fact that the windmilling prop (remember that Vmca is measured with the prop windmilling) blanks out the gear - the amount of extra drag when the gear is lowered is less than the amount of extra drag when the gear is raised. Difficult to explain without pictures, but there's a good explaination with a picture
here.
Depending which of these factors overrides the other, Vmc may either increase or decrease when the gear is extended, depending on aircraft type. In any case, the effect will be incredibly small - so much so, it's not even mentioned by many text books.
It should be noted that Vmca is measured with the gear up. This is because it reflects the take-off scenario - if the gear was still down and the speed was low, closing both throttles and landing ahead would probably be the best course of action, terrain permitting. It should also be noted that none of this is at all relevant to the original question!
FFF
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