Most VR technologies involve the visual sense, which was the case when I was playing with it. In my opinion immersion should also address the other senses: auditory, olfactory, tactile and taste. Exposure to all sensory experiences could possibly induce psychological effects or experiences. Today’s VR, however, covers the visual and the auditory senses only, and with the arrival of haptics, a semblance of touch is possible. I would think you would need a range of instruments and measures, if your intention is indeed to measure imnmersion.