When I set duckduckgo's onion service as my default browser, vanadium just chooses to use duckduckgo.com instead of https://duckduckgogg42xjoc72x3sjasowoarfbgcmvfimaftt6twagswzczad.onion
My guess is that this has something to do with both search engines being listed with the same name. (duckduckgo.com is listed first)