We have made reliable measurements of the sound velocity δv/v0 and internal friction Q-1 in vitreous silica at 1.03, 3.74, and 14.0 kHz between 1mK and 0.5K. In contrast with earlier studies that did not span as wide a temperature and frequency range, our measurements of Q-1 reveal a crossover (as T decreases) only near 10mK from the T3 dependence predicted by the standard tunneling model to a T dependence predicted if interactions are accounted for. We find good fits at all frequencies using a single interaction parameter, the prefactor of the interaction-driven relaxation rate, in contrast to earlier claims of a frequency dependent power law. We also show that the discrepancy in the slopes d(δv/v0)/d(log 10T) below and above the sound velocity maximum (1 -1 observed, 1-2 predicted) can be resolved by assuming a modified distribution of tunneling states. © 2008 The American Physical Society.