If you comment out the entire file clientscript/vault/section.js, the problem persists so it is not related to this file. It seems to go away if you comment out the entire wiki CSS file. This makes absolutely no sense, since the button node's existence shouldn't be affected by any style changes. As this doesn't occur in IE9, Firefox 4, or the latest Chrome, I believe that this is a problem with the rendering engine in IE8 and other browsers that complain about this. There's nothing we can really do about it, since the VaultWiki CSS doesn't even reference the classes used by the buttons...
Attempting to debug this from the plugin.js file is impossible, because in the caller you reach an implicit sanity failure. You cycle through objects (that must exist to cycle through them), run a function on one, and inside the function it stops existing.
I have noticed that there is a button in IE8 that I don't see in other browsers, which may be triggering the problem. IE8 has a Spell Check button, and what do you know, IE8 in 64-bit mode doesn't have Spell Check capability. However, even after installing ieSpell, it's not possible to use it in either 32- or 64-bit modes.