It’s impossible to say what is going on. Here is what I think is happening. Your LCD or Viewfinder on your camera is giving you bad information. It’s probably set too bright. This would explain why when yo uprint to tape it looks closer to what you thought you had.
The only way to tell for sure is to run the playback video out from you camera into a waveform monitor and check the levels that are coming out of you camera.
You need to check this against the waveform monitor in Vegas. This will also tell you if there is a problem with the monitoring out of your computer.
If you do not have access to a waveform monitor. Record color bars in your camera and then bring them in firewire to Vegas. Check the levels on the wave form. The peak of level from the colorbars should be around 100% give or take a few IRE.
If this is what you see then the viewfinder/LCD in your camera is too bright and the camera is not really recording what you see. This is a common prblem for anyone who shoots with LCD’s.
Dan