I too have the problem of dark renders, created correcting for what I believe is extra brightness in the Video Preview window. Various forums have solutions that didn’t work for me, ..but I found a work around by accident. ..which I think may show a flaw or vagary with some systems, in Sony Vegas Platinum. ..mine is ver. 11.
In short, brightness and color match closely in all these: Orig clip, icon shown in Media area, and icon on timeline. The Video Preview window is noticably brighter. My two monitors match when moving a clip between the two. I happened to discover that, if, in the Video Preview window, I click on the External monitor icon (blue, upper left corner, the video jumps (still on the second monitor) to full screen, and changes to proper brightness.
In other words,choosing a slightly different means for displaying the Preview, changed the brightness setting substantially. …seems to me a Sony Vegas problem many are having, that none of us understand.
…any words of wisdom? Thanks.