It's fine to install both, and you are correct that the other webview you install will not be usable. Only Vanadium's WebView can be used on GrapheneOS, nothing else.
I very much doubt that OkCupid is actually checking what WebView is being used and even if it's enabled. Disabling it should work fine, as it likely just checks for whether an app with that app ID is installed. It's possible that they also do a signature check to confirm it's the official Android WebView rather than one masquerading as it by re-using the same app ID.