Cracked heels occur when there is a build up of callused skin that is hard and it just ‘çracks’. Once that hard skin has cracked, it will not heal as it is not living tissue and that crack will put pressure on the underlying skin and could tear that, becoming painful and serving as an entry point for a potential infection.
Under the heels is a fat pad that is designed to help cushion the foot. This shock absorbing pad expends out sideways under the heel bone when we strike the ground when walking or running. To allow for that expansion, the skin has to be supple and flexible. If the skin becomes dry and, more importantly, develops the thick hard callus then the skin is not going to be able to absorb that fat pad expansion and will simply split or crack from the forces.
The callused skin is due to too much pressure on that area, most likely from the way the foot moves when you are walking.
Cracked heels are not caused by a Vitamin deficiency. There are various strategies that can be used to fix cracked heels, including the use of a good foot cream to keep the skin supple and flexible and a foot file to have a go to remove the hard thick skin. Or simply go to a podiatrist and get it fixed and discuss options for the long term