I think not zero sum in the sense that currently a good battle between two rivals could stir up more attention for both VR companies and lead to increased sales

I think not zero sum in the sense that currently a good battle between two rivals could stir up more attention for both VR companies and lead to increased sales for either than they would have if they weren't competing. People don't have to buy both they just have to be more likely to buy either due to the presence of competition.