BCC Iron is stiffer than FCC iron because of the crystal structure. Vacancies are point defects in crystalline materials. Vacancies are missing atoms from a lattice crystal structure. The number of vacancies and is often determined by the temperature of the material. The higher the temperature the higher the concentration of vacancies in a solid.
Vacancies can often be responsible for creep and other failure mechanisms at higher temperatures.
Vacancies also allow for impurity atoms to exist in a lattice, which can change material properties. Surface defects like cracks are formed when two surfaces separate. That is the bonds between the atoms in the two planes break. The formation of these surfaces is associated with a surface energy. The surface energy can then be related to the fracture toughness.