What you can do is this: when you configure your widget, near the bottom is a field for Template Name. It should say "vbcms_widget_execphp_page". Change that to something like "vbcms_widget_execphp_guests_only".
Then go to the Style Manager and (probably in your default style) edit the vbcms_widget_execphp_page template and copy the code. Then create a new template called vbcms_widget_execphp_guests_only and paste in the code, and add 'if' tags around it, like:
Code:
<vb:if condition="$bbuserinfo[userid] == 0">
// existing code here
</vb:if>