Try something like, in vbcms_content_article_preview, find and remove:
<vb:if condition="$showpreviewonly">
<vb:if condition="$previewvideo">
{vb:raw previewvideo}
<vb:elseif condition="$previewimage" />
<a href="{vb:raw page_url}">
<img class="cms_article_preview_image" src="{vb:raw previewimage}" alt="{vb:rawphrase article_preview}" /></a>
</vb:if>
</vb:if>
And then under this:
<div class="cms_article_txt_content postcontainer">
{vb:raw previewtext}
<vb:if condition="$preview_chopped">...</vb:if>
</div>
Add this:
<p class="bottom_image">
<vb:if condition="$showpreviewonly">
<vb:if condition="$previewvideo">
{vb:raw previewvideo}
<vb:elseif condition="$previewimage" />
<a href="{vb:raw page_url}">
<img class="cms_article_preview_image" src="{vb:raw previewimage}" alt="{vb:rawphrase article_preview}" /></a>
</vb:if>
</vb:if>
</p>
And add to additional.css:
.bottom_image {display: block; clear: both; float: none; text-align: center;}
.cms_article_preview_image, .showpreviewonly object {float: none;}
You'll probably want to tweek it a bit to be how you want, but that's the basic idea.