When Riot Games introduces the Vanguard anti-cheat to League of Legends, you should STOP playing and you must NOT install the anti-cheat when you get the pop-up. Vanguard is a kernel-level anticheat and these anticheats operate at a privilege level HIGHER THAN YOUR OWN. The anti-cheat can do things that even YOU can't do, without asking or letting you know. It's like Riot installing a camera in every room of your house and getting a copy of every key inside.
Here are just a few examples of what they can do: