Cavitation bubbles collapsing and rebounding in a pressure gradient del p form a "microjet" enveloped by a "vapor jet." This Letter presents unprecedented observations of the vapor jets formed in a uniform gravity-induced del p, modulated aboard parabolic flights. The data uncover that the normalized jet volume is independent of the liquid density and viscosity and proportional to zeta equivalent to vertical bar del p vertical bar R-0/Delta p, where R-0 the maximal bubble radius and Delta p is the driving pressure. A derivation inspired by "Kelvin-Blake" considerations confirms this law and reveals its negligible dependence of surface tension. We further conjecture that the jet only pierces the bubble boundary if zeta greater than or similar to 4 X 10(-4).