I just took the code from my product -
Tag Cloud on Forumdisplay and added it into the page. You have stuff in the products that adds to the $phrasegroup and $globaltemplates. Then add all the code from the forumdisplay_start plugin except get rid of the whole condition around it. Then, make sure to set $foruminfo[forumid] to 0 because or else you will get an error and then get rid of anything that has to do with the $forumid - in the query and when setting the tagtest_urls (two places). Then, in your php page, make sure to register $cloud_html for your template ($templater->register('cloud_html', $cloud_html); ) and enter the variable into the template ({vb:raw cloud_html}). That should work.