  Based on density functional theory the noncentrosymmetric superconductor
Mo_3Al_2C in its well established {\beta}-Mn type ($P4_{1}32$) crystal
structure is investigated. In particular, its thermodynamical and dynamical
stabilities are studied by calculating lattice vibrations within the harmonic
approximation. It is found that the fully stoichiometric compound is
dynamically unstable. However, compounds with carbon vacancies, i.e.,
Mo_3Al_2C_{1-x}, can be dynamically stabilized for vacancy concentrations x >
0.09. By means of a simple thermodynamical model we estimate x ~ 0.13-0.14 for
Mo_3Al_2C_{1-x} at the experimental preparation temperatures. The influence of
the carbon vacancy concentration on the electronic structure is investigated.
