LogFAQs > #980196602

The gist of it is that it is always running and making connections and sending encrypted data. It runs at the Kernel level. While other games like Counter Strike have anti-cheat at the Kernel level, you can turn them off and they only do things when you play the game and do less things. This is always doing stuff and if you turn it off you have to reboot the computer to play Riot games again.
See synopsis by The PC Security guy in the video.

https://www.leagueoflegends.com/en-gb/news/game-updates/patch-14-9-notes/

With 14.9, Vanguard, Riots proprietary Anti-Cheat system will be deployed and active in League of Legends. This means that active enforcement of Vanguard will be in effect and working hard to make sure your queues are free from scripters, botters, and cheaters! We recently released a blog detailing the why behind bringing Vanguard to League that you can check out here. Its a bit of a long read, but it does have some pictures.

If youre wondering what all you need to do to keep playing League, all you need to do is install Vanguard. If you do not have Vanguard installed when you launch the Riot Client or League it will prompt you to install it and then restart your system to ensure Vanguard is active. You will be unable to queue for or play League until Vanguard is installed and active.

If you run into any issues please check out our support articles for any troubleshooting needs involving Vanguard

https://www.youtube.com/watch?v=UqLI1xKc-L4