It probably uses some javascripts from one of the episerver VPP:s, ~/Util/javascript/common.js etc. So for it to work in your page you must have a reference to these javascripts (it could be that the edit panel loads them so they are already available in the preview pane). The user must also have access to these scripts, which are by default only available to the editors since they reside in a protectd location (/Util etc). This could also be the reason that it works in edit mode but not in view mode (if you are not logged in in view mode). Add a new, more specific location for the javascripts you need and enable anonymous access to it.