Hi all, someone on the Valorant discord posted the solution, so thought I'd share it here:

Apparently something in the new patch removed Valorant as an exception in SOME antivirus softwares (AVAST, Malware Bytes). So, you need to re-set it as an exception. I also had to set the entirety of the 'Riot Games' folder as an exception.

Good luck :)

EDIT: You might also be able to solve this by instead adding adding additional exceptions to your Firewall. Thanks to ahbooyou for flagging this link: https://support-valorant.riotgames.com/hc/en-us/articles/360048522893-Your-Firewall-VS-VALORANT

I just followed the steps there (I already had Valorant itself as a Firewall Exception; but needed to add 'C:\Program Files\Riot Vanguard\vgc.exe' and 'C:\Riot Games\Riot Client\RiotClientServices.exe' also). Once I did that, I removed the anti-virus exceptions for the Riot Games folder and Valorant from Avast, and my game's still booting :)