Question

How are vacancy defects stoichiometric defects if some atoms are absent? Wouldn't that change the stoichiometry of the compound?

Answer 1

The defects that do not disturb the stoichiometry (the ratio of numbers of positive and negative ions) are called stoichiometric defects.
But in the case of vacancy defect a complete atom is missing, since the number of ions are still the same and equal therefore it is a type of stoichiometri defect.