Since Vitamin D helps with insulin sensitivity, it makes some sense to me in theory that we would be able to tolerate more carbs the closer to the equator we lived (or by having an equally high D level.)
I've seen some studies indicating that it might help with weight loss. Correlation though, not necessarily causation. Essentially along the lines of "everyone with sufficient D levels didn't gain weight, some people with low D levels gained weight". Nothing more firm than that, and it doesn't directly relate to carbs. The theories on it seem to be based around Vitamin D possibly downregulating the creation of new fat cells. I don't recall seeing anything saying Vitamin D boosts metabolism.