A theoretical model of a multimode vector class-A linear laser with two sets of anisotropic elements on both sides of an active medium has been proposed. In this case the polarization and intensities of counterpropagating travelling waves of each mode differ. On the basis of the theoretical model presented, single longitudinal mode emission of a vector class-A linear gas laser has been explored. Numerical results predict the existence of three stable steady states in a certain domain of the control parameters such as detuning and phase anisotropy as well as the hysteresis phenomenon with spontaneous polarization symmetry breaking and its restoration. As is shown it is necessary to take into account the joint influence of the resonator and the active medium on the parameters of the emitted modes and their corresponding contributions to the laser equations.