Browser fingerprinting is identifying a user by inherent information about their environment that is available to the server even without cookies. Stuff like location, OS, browser settings (font size, version, etc) and anything else that they can use as a data point. Combining all of these variables can sometimes be unique enough to identify individual users, within certain contexts.

I don't know what Vanadium does to protect against this, the usual privacy recommendation is to not adjust any browser settings for personalisation (e.g. dark mode).