Thermodynamic data with regard to slags containing vanadium oxide are scarce and scattered. In the present study, the classic gas-slag equilibrium technique has been adopted to investigate the phase relationships in the CaOSiO2MgOAl2O3V2O3 slags. The phase equilibria in the slag systems in the temperature range of 1773–1823 K (1500–1550 °C) at oxygen partial pressures of 10−5 Pa and 0.21 × 105 Pa were characterized. CaVO3 was precipitated in samples with basicity 1.5 and 1.8 treated in air at 1773 K (1550 °C) while all vanadium was dissolved in equilibrium phases in samples treated at oxygen partial pressure of 10−5 Pa. Vanadium distribution between the slags and solid Pt and phase compositions were analyzed using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray diffractomer (XRD) techniques. The effects of temperature, oxygen partial pressure, and slag basicity on the phase compositions in the slags are presented. Activities of VO1.5 were also calculated using the value of vanadium activity in the Pt–V alloy.