Yes, it's legal to use a VPN with Valorant. But, countries like China, Belarus, Russia, U.A.E., Oman, Iraq, and North Korea have either completely banned or regulated VPN usage.
So, if you're playing Valorant from any of these countries, you need to use a premium VPN service like ExpressVPN or NordVPN that can make you go completely anonymous no matter where you're located. Such VPNs hide your online traffic even from the network admin and your ISP.
Hence, no third party will ever have knowledge of your online activity, and you'll never be charged for playing Valorant. Moreover, if you're not a permanent resident and just traveling in these countries, using a VPN will never bring any trouble.