How can a vacancy defect be a stoichiometry defect? NCERT (India) explain stoichiometric defects as those defects which do not disturb the stoichiometry of the solid. Vacancy defects are stated to be under the category of stoichiometric defects. When some lattice sites are vacant, the crystal is said to have a vacancy defect.

My question is: if lattice points are missing in vacancy defects, shouldn't this disturb the stoichiometry of the compound? I think yes but the book states no. Why?
Yes, you are not completely wrong but try to answer these questions to check your understanding (mouse over for the answers):

First of all, what is a Non-stoichiometric compound?


   Non-stoichiometric compounds are chemical compounds, almost always
 solid inorganic compounds, having an elemental composition whose
 proportions cannot be represented by integers.


In which way a vacancy affects the ratio of the elemental composition?


   If the compound it's a "pure" metal the vacancy will not affect at all the ratio of the elemental composition. However, it could affect the ratio if, for example, we have $\ce{NaCl}$ crystal where a lot of $\ce{Na+}$ ions are missing.


How is likely to have an ionic compound where a lot of anion or cation are missing?


   Not at all, the ionic compound, usually, maintain an overall neutral charge see Schottky defect.