Note: I have also posted this at vbulletin.org
In vBulletin 4, the developers recently upgraded the bundled YUI version to 2.9.0 from 2.7.0. Since vB 3.8 is EOL, I found a way to manually update the bundled YUI version. I’ve tested vB 3.8 with YUI 2.9.0 as much as I could, and did not run into any problems.