We propose an experimental protocol for the direct laboratory test of the hypothesis of a fundamental yield threshold of the physical vacuum — a critical value of gravitational self- energy Emax ≈ 2.05 × 10−32 J, at which the reversible geometric deformation of spacetime (GR metric) abruptly transitions into irreversible plastic dissipation. The protocol relies on ringdown measurements of the center-of-mass oscillations of levitated fused silica (SiO2) microspheres in an optical tweezer at cryogenic temperatures. The primary marker of the transition is a discontinuous drop of the quantum contrast V of the interference pattern. A precision scanning grid of eight steps in the diameter range 1.0–5.0 μm is constructed, with the critical point Dcrit = 2.863 μm and the critical mass Mcrit = 27.04 pg. Rigorous mathematical criteria for confirmation and falsification are formulated, including a temperature-invariance test and cross-material scaling. The model is falsifiable: the predicted discontinuity in V(M) is experimentally distinguishable from the smooth monotonic decay expected in thresholdless collapse models (GRW, CSL, Diósi–Penrose).