When
got his visual update,they made a small devblog about updating the skin splash arts of the champs that got already updated and said that they want to give every champ who received a VU new skin splash arts.
They later clarified here on the boards that when they started the project to update skin splash arts,they did it kinda unorganized(thats why we got new vandal brand,assassin&chosen yi and jade dragon wukong splash),but now they focused on the updated champions and chose
and
as their top priorities.
Trundle because there is a very big difference between his old skin splash arts and their models
Tristana because Riot girl tristana is the first skin for many people
Nidalee because when the skin-splash art project was unorganized they did already a lot of work on Nidas splash arts,so Nida is already rather close to completion
However they also said that they'll release updated skin splash arts for each of these champions at once,so we'll have to wait till all the tristana splash arts are done(though the ones of the others can come in different patches)
EDIT:
Source
http://boards.na.leagueoflegends.com/en/c/story-art/Iss0zpAe-dev-blog-updates-and-splash-art?comment=00000001