On a different note: We don't need to bring in quantum mechanics ("quantum vacuum") here. General Relativity is a purely classical (i.e. non-quantum) theory an

On a different note: We don't need to bring in quantum mechanics ("quantum vacuum") here. General Relativity is a purely classical (i.e. non-quantum) theory and doesn't care about quantum mechanics. So the vacuum we're talking about here is the "classic vacuum".