[Velocd]

Let me ask this first:
Why are you trying to include an HTML file? Just do whatever your doing in the HTML within a template, and then use dooutput in the index.php file to output the template, when called.

PHP Code:

 if($action == "main") {
  eval("dooutput(\"".gettemplate('main')."\");");
} 


Create a new template called main, then put the stuff you would put in that HTML file in that template. No need to use .html files, when you got vBulletin template system