missing-root Correct. You can add any search engine which implements OpenSearch.
You can add Startpage as a search engine in Vanadium. Before you conduct a search (also make sure it's not in incognito mode), you have to go into the settings of Startpage and change the HTTP request method from "POST" to "GET." Conduct a search and you should see it show up in the list of search engines.
Mullvad Leta is accessible only with a paid Mullvad VPN account, so that most likely would cause an issue adding it as a search engine within Vanadium.