I believe the preview is stored in:
$view->previewtext and the hook you need is
vbcms_article_populate_end.
At least that is how I manipulate the preview in my mods.
The php function strip_tags should remove HTML tags so just make a new plugin on the hook above with this code:
PHP Code:
$view->previewtext = strip_tags($view->previewtext);
See if that does it.
Off hand I know my "Word Linker" mod has code on this hook if you want to see a real example.