If your in-software download fails or is slow, Chaos Group provides a direct HTTPS link for manual download. As of V-Ray 6 (Update 1 and 2), the official link is:
https://download.chaosgroup.com/materials/vray
Note: This URL redirects to the latest versioned file (usually a .exe for Windows or .dmg for Mac). You do not need to pay for this; it is free with any V-Ray 6 license.
Once downloaded, run the installer. It will automatically place the files in the correct directory.
/home/[Your Username]/v-ray material library
A: Yes, 100% free. You do not need an extra subscription. It is included with every V-Ray 6 commercial license, educational license, or trial.