When　the　mechanical　oscillator　is　initially　in　an　excited　state,　its　free　evolution,　driven　by　the　dynamical　Casimir　effect　(DCE),　leads　to　the　generation　of　radiation　in　the　vacuum,　analogous　to　the　spontaneous　emission　of　an　excited　atom.　In　the　regime　where　the　loss　rates　are　lower　than　the　corresponding　frequency　splittings,　reversible　energy　exchange　can　occur　between　the　excited　mechanical　oscillator　and　the　vacuum　cavity　field　(i.e.,　vacuum　Casimir-Rabi　oscillations).　By　using　the　quantum　trajectory　approach,　we　show　that　n-photon/n-phonon　emission　can　be　achieved　by　opening　dissipative　channels　for　these　Casimir-Rabi　oscillations.　Simulation　results　from　105　quantum　trajectories　demonstrate　that　the　purities　of　two-photon　emission,　two-phonon　emission,　and　three-phonon　emission　can　reach　0.92,　0.94,　and　0.95,　respectively.　These　values　are　observed　in　Casimir-Rabi　oscillations　between　the　three-phonon　state　and　the　two-photon　state　in　a　weak　optomechanical　coupling.　This　phenomenon　could　provide　insights　into　the　physical　characteristics　of　photon　and　phonon　emission　in　the　DCE.　©　2025　Optica　Publishing　Group　under　the　terms　of　the　Optica　Open　Access　Publishing　Agreement.