Some time ago I was using Vegas Pro 11 as the video editing software, and later switched to Adobe Premiere Pro for reasons I no longer remember. I then switched back to Vegas Pro (the latest version) because I found it much easier to use, but then noticed that videos are darker when rendered to a container other than an AVI/uncompressed AVI.
I've done some research and it seems that it's an actual "problem/feature" that has a workaround by levelling the colors (Computer RGB to Studio RGB), but I feel like it doesn't give the original color to the final output. Other posts say that Windows Media Player and YouTube alter the colors, while other video players show the color the original media has.
The thing is that I don't remember having this issue on older Vegas Pro versions, nor with Premiere Pro (I'm pretty sure with this one). A third post suggested rendering the video to some kind of a container and then re-encoding it on Handbrake, but I once gave Handbrake a try and all it did was give me "worse" quality results after comparing two same MP4 samples (one rendered with Vegas, the other with Handbrake - both from an AVI).