Primeiro, você precisa instalar o mecanismo de pesquisa em: link
Depois, você pode definir o mecanismo de pesquisa padrão usando:
set defsearch=duckduckgo
No arquivo de ajuda:
'ds''defsearch' string(default: google)
Sets the default search engine. The default search engine is used by :open and related commands for arguments which include no search or bookmark keywords and can't otherwise be converted into URLs or existing file names.
This means that with 'defsearch' set to youtube, :open Tim Minchin behaves exactly as :open youtube Tim Minchin, so long as you don't have a search or bookmark keyword called ‘Tim’.