Simple bit of CSS here to hide the header on your forums (Logo and Search). I have attached a screenshot to show the result.
Add this to additional.css:
#header {display: none;}
What did you do with the search box?
I have to ask "Why" would you want to hide the Logo, isn't that your sites identity?
It was made for someone who wanted quite a number of changes to their vBulletin system, they were using it in an Intranet environment so it wasn't exactly needed for their specific needs. I put this here in case anyone else wanted to do something similar. Nothing more :).