I'm not sure what you mean. The way I understand it, the vacuum is capable of creating arbitrary particles as long as the quantum numbers are conserved and ΔEΔt

I'm not sure what you mean. The way I understand it, the vacuum is capable of creating arbitrary particles as long as the quantum numbers are conserved and ΔEΔt >= ħ/2. With e- being the lightest (apart from neutrinos, which are negligible), the vacuum is producing virtual e-/e+ pairs, which are long lived enough to interact with real particles. As an approximation, the vacuum is exclusively an e-/e+ soup, since the other quarks and leptons require much too large of a ΔE to instantiate. The way I understand γ-thrusters is that they make use of the very tiny amount of momentum in a single photon, p = ħk, along with the large N of them bombarding the object. I haven't seen any calculations for γ-thrusters that take into account vacuum fluctuations, though.