Code not working

[Itworx4me]

Hello,

I am trying to replace the username with a profile field. Trying to get the following code to work as a plugin. Can some look it over and see why it doesn't work?

if ($vbulletin->userinfo['usergroupid']=='1')
{
  $output = str_replace('$post[musername]', $post[field2], $output);
}

Thanks,
Itworx4me