Precise low-frequency internal friction measurements on vitreous silica, taken over a wide temperature (4K < T < 300 K) and frequency range (40 Hz < nu < 14 kHz), show remarkable similarities, but also suggestive differences with recent light scattering experiments. In the interval 30K < T < 110 K, the exponent alpha of the power law relaxational spectrum turns out to be proportional to T-T(k), with a minimum at around T(k) = 13K. This phenomenon is interpreted as a manifestation of thermal activation in an asymmetric double well system, the asymmetry being associated to quenched spatial disorder. For T > 160K the loss angle develops a distinct step-like structure followed by a plateau, both independent of nu, thus signalling the onset of a competing relaxation mechanism with much higher an activation energy. Copyright (C) EPLA, 2007.