Atomically strong light pulses can drive sub-optical-cycle dynamics. When the Rabi frequency-the rate of energy exchange between light and matter-exceeds the optical carrier frequency, fascinating non-perturbative strong-field phenomena emerge, such as high-harmonic generation and lightwave transport. Here, we explore a related novel subcycle regime of ultimately strong light-matter interaction without a coherent driving field. We use the vacuum fluctuations of nanoantennas to drive cyclotron resonances of two-dimensional electron gases to vacuum Rabi frequencies exceeding the carrier frequency. Femtosecond photoactivation of a switch element inside the cavity disrupts this 'deep-strong coupling' more than an order of magnitude faster than the oscillation cycle of light. The abrupt modification of the vacuum ground state causes spectrally broadband polarization oscillations confirmed by our quantum model. In the future, this subcycle shaping of hybrid quantum states may trigger cavity-induced quantum chemistry, vacuum-modified transport or cavity-controlled superconductivity, opening new scenarios for non-adiabatic quantum optics. Deactivation of deep-strong light-matter coupling is achieved by femtosecond switching of terahertz cavities. This disruption leads to pronounced high-frequency polarization oscillations evolving much faster than the oscillation cycle of light.