On the exact definitions of Frenkel and Schottky defects
Well, you can say that they are kind of similar...the only major difference is that Schottky defects are only for ionic compounds, and the number of missing cations and anions must be equal. Vacancy and interstitial defects can be shown by non-ionic solids. Ionic solids must always maintain electrical neutrality. Rather than simple vacancy or interstitial defects, they show these defects as Frenkel and Schottky defects.
Vacancy Defect: When some of the lattice sites are vacant, the crystal is said to have a vacancy defect. This results in a decrease in the density of the substance. This defect can also develop when a substance is heated.

Schottky Defect: It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal.
Like a simple vacancy defect, the Schottky defect also decreases the density of the substance. The number of such defects in ionic solids is quite significant. For example, in NaCl, there are approximately 106 Schottky pairs per cm3 at room temperature. In 1 cm3 there are about 10^22 ions. Thus, there is one Schottky defect per 10^16 ions. The Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects.

Also, with respect to your comment: &quot;I thought Frenkel defects weren't vacancy defects but classified separately under interstitial defects.&quot;
Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substances in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and AgI due to the small size of Zn2+ and Ag+ ions.

Interstitial Defect: When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have an interstitial defect. This defect increases the density of the substance.

Also, here's why AgBr shows both Schottky and Frenkel defects, even though Frenkel defects are shown by ionic substances in which there is a large difference in the size of ions, and Schottky defects are shown by ionic substances in which the
cation and anion are of almost similar sizes.

Schottky defect in AgBr is exhibited due to precipitation of both cations and anions. In AgBr, Ag+ ion is small in size and when removed from lattice point they can occupy interstitial sites and hence show both Frenkel and Schottky defects.