I prefer nami x vivi, but fine. Seems like One Piece yuri is super rare for some reason.
I don't think it's that hard to figure out, there are way more male characters (it being a shounen series). Despite it's massive popularity, it doesn't seem to be a popular target for doujinshi, much less yuri.
I think because most fans want to see the development of the story and the characters goals and dreams rather than romance. Maybe because One Piece fans are mostly younger people.