Abstract Equilibrium configurations in L1 2- and B2-ordering ternary alloys A 3B 1 − x C x and AB 1 − x C x (0 ⩽ x ⩽ 1) containing thermal vacancies are studied within the Bragg-Williams approximation. A new approach proposed for the estimation of vacancy concentration in a crystal consists of analysing a state of equilibrium with respect to vacancy migration between the crystal and its surroundings considered as an infinite “vacant” crystalline lattice. Direct numerical minimisation of the alloy free energy yields a full overview of an interplay between vacancy formation, vacancy ordering and atomic ordering in alloys characterised by particular interrelations between pair interaction energies of atoms. It is found out that, without affecting the character of chemical ordering, various relations between atomic pair interaction energies control the vacancy formation process and lead to qualitatively different vacancy configurations. The origin of considerable discrepancies between the present results and some of the literature data is discussed.