I've never heard (or heard of) this behavior in Vegas. Is the audio fed directly to the PC's sound card on location, or to an external recording device (camera or audio recorder) and the files are opened in Vegas? When you say "too loud", does that mean clipped? Clipping (preamp and/or digital overs) distorts audio, and in severe cases can make it difficult or impossible to understand. Post production peak clip restoration and/or limiter plug-ins, won't help much if it's severe.