I think (based on this official post) that the question might be based on a misunderstanding. If I am understanding correctly, running on-the-web tests against Vanadium is testing Vanadium, not testing the WebView component, which is used internally by some apps. It's not clear to me how one would test WebView itself for privacy issues.
Overall, I think if you prefer the results that online privacy tests assign to browsers other than Vanadium then it may make sense for you to install and use browsers other than Vanadium. But you might want to consider other factors as well, such as whether a browser that is assigned a high privacy score is keeping up with security patches. GrapheneOS doesn't force users to browse with Vanadium, so it's up to you.