This is an old thread but whatever.
There are more ways to get VP9 than to upload in 1440p.
So, here are the known ones;
Rendering in 1440p
Lots of views
(?) Lots of subscribers
Some people just randomly get VP9 on their videos.
Here are the "secret" / "unknown" ways to get VP9.;
(KIM) You don't need to render in 1440p or have a lot of views.
1. Go to "Video Manager," find your video, click "Edit", click on the "Enhancements" tab, and drag the little preview slider all the way to the left. You don't touch anything else, just drag the slider to the left and click save changes. Your video should get VP9 soon.
2. Enable Google Drive for your channel if it's not already. Go to Google Drive, click "Upload" and select the rendered video or just drag and drop into the general area, when it's done uploading, go to your YouTube, click "Upload", in the top right there should be something that says "Import Videos", click "Import", find your video and select it, when the video is done processing it will have VP9 in a little bit.