Q:

Different luminosity between preview and render on Sony Vegas 13

I currently have a problem with a video I'm editing, when I render it with Sony Vegas 13 (with the Sony AVC/MVC 1080p-30fps "Internet" preset), the resulting video is much darker than the preview (on the same screen), whatever player I use (VLC, Youtube, Windows Media Player). 
This is really annoying because I now have to export small chunks of the video to check the resulting luminosity without being able to preview it properly. 
I already did dozens of video with the exact same process (same camera, same lighting setup...), except I was using version 12, and I currently can't try to revert back to previous version. 
Any idea of what's happening ? 
Any suggestion of what I can do to have a preview that matches the rendered video ? 
Some context :
The video is a man wearing black standing in front of a green screen.  The green screen is keyed to a complete black screen. 
In order to be able to see the man's body, I adjust the luminosity and contrast so that the clothes are slightly brighter than the background. 
After rendering, everything is darker so the clothes are of the same color as the background and you just see a face with hands floating...
UPDATE
User altarvic suggested that I check the histograms of both videos and to do that I imported back the rendered video in Vegas.  And there, the luminosity is correct in preview (the image is exactly what the preview displayed). 
So it seems that the luminosity is wrong on players (at least the ones I tried), but not in Vegas. 
Could it be a codec issue ? ( Vegas using it's own, player using the machine's). 
Or the Vegas preview is not displaying properly the video (I checked I have no output effects applied). 

A:

I believe I have a solution.  Render the video, put it into Vegas again and apply "Sony Levels" with "Computer RGB to Studio RGB".  Worked for me.