Quantum vacuum is an effective way to measure the absolute value of a physical quantity. In the case of a superluminal particle, a vacuum is a vacuum that can be made without any loss of charge. There are several ways to measure vacuum for quantum effects.
One of them is by measuring the position of a particle at the speed of light. But quantum vacuum is really just a way to measure the absolute value of the quantum vacuum, which is a property of a quantum object.
