Oh, I was thinking you wanted it on the article page, not on the section page. The section page would be template vbcms_content_article_preview, for the individual previews, but I think if you are listing them in one column, then it is template vbcms_content_section_type1 that spits them out. If you look in that, you can see this:
HTML Code:
<vb:if condition="$id != 1"> cms_separator</vb:if>
So, it looks like you can use a condition based on the id to add something there - if $id == 1, then add ads.