I emailed Johnny regarding this, but thought I would post just in case someone else had the problem.
Yesterday I loaded Vegas and my UltimateS tab was gone from the "Explorer" area - where it has been since I originally bought 13. It seems like some other settings, like the layout of the windows, was changed as well. Crazy.
If you reload Ultimate S from the menu, you should be able to redock it. I usually have Ultimate S as a shortcut icon amongst shortcuts the cuts and fades (as I use Quadcam a lot). I have noticed that if I minimise Vegas or switch to another app for a while, these icons disappear when Vegas is made the main app, but restarting Vegas reinstates them. They are still there because you can click on them, just the buttons are not visible. It's a liveable quirk.
My system specs: Intel i7 970, GTX570, 12GB RAM, ASUS P6T, Vegas Pro 12 (x64), Windows 7 x64 Ultimate, Vegas Production Assistant 1.0, VASST Ultimate S Pro 4.1, Neat Video Pro 2.6