A new experimental technique is reported that provides information on vibrational quenching of polyatomic molecules in collision with surfaces. A bolometer detector is used to measure angular distributions for the ground state and the survival probability of vibrationally excited states of acetylene. We find that for LiF(100) there is essentially no vibrational quenching in the direct scattering channel. Even when molecules are trapped for significant periods of time, such that their translational distributions are fully accommodated with the surface, vibrational quenching can be small. Under extreme conditions, however, where the trapping times are very long, we observe appreciable quenching of the vibrationally excited molecules.