We report theoretical and experimental analysis of spin-injected VECSELs. First, we fabricate and characterize an optically pumped (100)-oriented InGaAs/GaAsP multiple quantum well VECSEL. The structure is designed to allow the integration of a Metal-Tunnel-Junction ferromagnetic spin-injector for future electrical injection. We report here the control at room temperature of the VECSEL polarization using optical spin injection in the active medium. The switching between two highly circular polarization states had been demonstrated using an M-shaped extended cavity in multi-modes lasing. This first result witnesses an efficient spin-injection in the active medium of the laser. Then, we report birefringence measurements of the VECSEL in oscillating conditions. The proposed technique relies on the measurement in the microwave domain of the beatnote between the oscillating mode and the amplified spontaneous emission of the cross-polarized non-lasing field lying in the following longitudinal mode. This technique is shown to offer extremely high sensitivity and accuracy enabling to track the amount of residual birefringence according to the laser operation conditions. Finally, we discuss the compensation of the residual linear phase anisotropy by controlling the birefringence of an intracavity electro-optical crystal. A 44-fold birefringence reduction is obtained. Besides, we study the modification of the laser polarization eigen states with birefringence compensation: a rotation of the linear polarization state is observed when the total phase anisotropy is reduced. An elliptical polarization eigen state is obtained at the minimum of the birefringence into the laser cavity, more favorable for spin injection. © 2016 SPIE.