I don't even remember anymore how to do this stuff
And I don't use VLC myself so I can't really comment on that, but
1) see if there is a screencapture button somewhere in the player rather than using print screen. But I guess print screen would do as well.
2) try taking some screencaps and compare them screencaps that you can find on this forum. See if they look identical. If not, let's try to figure out why. Some picture enhancement setting might be turned on.
3) also see if you can set the screencapture size from the player settings. Most of the old caps here are in 720 x 404 (anamorphic) / 720 x 540 (non-anamorphic) size, but these days you'd probably want to use a bigger size like 1024 × 576 (anamorphic) / 1024 x 768 (non-anamorphic) and of course full 1920 x 1080 for blu-rays. If you're making a comparison, try to make the caps the same size with the caps you are comparing them to.
I'm still using PowerDVD for most screencaps.