Start going through each link of the chain one piece at a time, comparing input to output. It could be any one of these devices or the signal chain as a whole.

To verify the levels are correct, add the Colour Input to vMix and look at it on the output.
Turn up the brightness on the monitor and if you can see two grey bars in the bottom right black section, then it is sending out 16-235 correctly.

vMix will auto detect 16-235 on both inputs and outputs to make sure it is correctly preserved. (It won't use 0-255 on broadcast signals)

It is possible for this to be overriden by the NVIDIA graphics under some rare circumstances, so checking
the NVIDIA Control Panel under "Adjust video colour settings" to be sure the brightness hasn't been turned down manually is the next step if the output test above shows things too dark.

This is all assuming that an SDI or HDMI output is being used on a supported capture card.
If you are using the HDMI out from the graphics card, then that level is likely the full 0-255 as that is what computer displays and projectors will expect under most circumstances.

Hope this helps.

Regards,

Martin
vMix