Apart from the fact that Brave doesn't add anything to GrapheneOS that Vanadium doesn't already offer in terms of security and privacy, and which already uses WebView, there's no need for redundancy, except for better ad blocking, but that's not the most important thing when it comes to security. On the contrary, I think Brave increases the attack surface with suspicious additions, even on desktop, I would recommend Google Chrome with uBlock Origin Lite (or without, as permissions such as reading website content have become more extensive by default) rather than Brave. Furthermore, the fact that Brave has partnered with Unplugged, a scam, to use Brave Search by default makes me even less inclined to recommend their products.