Try compression. Compression takes the quietest parts of the sound and boosts them, while cutting back on the loud parts to keep the audio in a certain volume range. Each track has a track compressor in its fx menu, or you can apply a compressor to the files themselves as media fx. you can also do multiple compressors in a chain, so play around until you get it. there's videos on youtube to teach you about what the settings do, and presets to play with.
Magix Wave Hammer would be a good choice for this. The compressor stage has an "Auto gain compensate" option which raises the level based on the threshold and ratio. The second stage limiter/maximizer can further boost the overall level. I do not recall if WH is included with Vegas. Is has been included with Sound Forge Pro as long as I can recall.