Like I said before it;s vbadvanced so I have no clue what hook you'd need, you can probably ask over at the vbadvanced forums. It may take a few days but Brian will usually answer in my experience.
Once you find the hook and the variable that holds the text of the preview, for example let's say it is $text, then you simply make a new plugin (ACP -> Plugin Manager -> Add new plugin) on the specific hook with this code:
PHP Code:
$text = strip_tags($text);
And that will output the $text minus any HTML tags.