What is the voltage of Vo in this circuit without a closed loop?
Indeed, you are correct about Vo = 0V because no current can flow through R2 and thus Vo is equal to GND.

Vo cannot be regarded as GND nor as virtual ground because there is an impedance between Vo and GND. True GND will not change if a circuit current changes. In this case however Vo will change if a current flows through R2 and Vo is therefore not identical to GND. Only in this single specific situation it happens to be at the same potential.

Virtual ground requires an active part that drives the virtual ground node in a circuit in such a way that it is kept at the same voltage potential as ground, which again isn't the case here.