How can a vacancy defect be a stoichiometry defect?
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.