View the source of the main search page, and basically remove elements you don't want, and be sure to point the forum to the right location of search.php if you put the non-vB page outside of the vB directory. The following code works for me for a general search (ie, all forums):
Code:
<form action="http://www.yourdomain.com/forum/search.php" method="post">
<input type="hidden" name="s" value="">
<input type="text" class="bginput" name="query" size="20">
<input type="hidden" name="forumchoice" value="-1">
<input type="hidden" name="titleonly" value="">
<input type="hidden" name="action" value="simplesearch">
<input type="submit" class="bginput" name="Submit" value="Perform Search" accesskey="s">
<input type="hidden" value="" name="showposts">
<input type="hidden" name="searchdate" value="-1">
</form>