There's some difficulties in implementing a toggle of that nature (I'm sure one of my more technical colleagues could speak to the effort involved, but an example that springs to mind is how to reliably tell a program to generally not launch on boot, except next time I reboot please do.")
I think if this is how you'd want to interact with Vanguard (which is great if it will help you feel authoritative about what you're comfortable with!), probably currently the avenue I'd suggest is to actually Uninstall Vanguard when you're not planning on playing LoL for a few days - it's also just a right click away (Right Click > More > Uninstall Vanguard > Confirm).
Then the next time you Launch LoL, Riot Client will prompt you to install Vanguard. Happily, the download + install is SUPER quick (my entire Riot Vanguard folder is 43 MB), so this flow actually wouldn't add much time to your get-in-game, as the reboot itself will probably be the longest part if you have broadband...
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