Did you try just adding them into the template after the headininclude template is called?
HTML Code:
<meta name="keywords" content="xxxx" />
<meta name="description" content="yyyy" />
and then change the condition in the headinclude template to not show those for your page:
HTML Code:
<vb:if condition="$show['threadinfo'] OR Your Condition">
<vb:elseif condition="$show['foruminfo']" />
other stuff....
<meta name="keywords" content="{vb:raw vboptions.keywords}" />
<meta name="description" content="{vb:raw vboptions.description}" />
</vb:if>