Vanguard takes screenshots of your PC every time you play a game. Every time you play a game a function is called to screenshot your PC’s screen, in case Vanguard thinks you might have something suspicious, it screenshots your ENTIRE PC screen (all monitors).
Edit: Not trying to spread false info this was shared to me via a friend and there is other data to back up that this is real https://www.unknowncheats.me/forum/anti-cheat-bypass/634974-vanguard-taking-screenshot-pc.html
https://www.unknowncheats.me/forum/valorant/484475-vanguard-screenshots.html
server side anticheat is the only ultimate solution.
all client side anticheat measures will have vulnerabilities that will be found and people will cheat. (look at DMA pci cards, for instance)
Also there’s a fundamental fog of war of what your anti-cheat can see. Valorant cheaters are using hardware cheats, that literally takes in a video output, analyzes it, and sends in mouse inputs, on a different computer, the anti-cheat straight up can’t see it, they only see there’s a video out and a mouse in. Ultimately, having physical access of a hardware and you can just tell the software whatever you want it to see.