Well you could do something like this:
For most short articles (summaries) templates are located in templates\xhtml\summary folder.
Now, to accomplish what you need, you must change all href's to go to external source.
For example, default.tpl template for summary:
<vte:template>
<div class="short">
<div class="short_holder">
<vte:if test="{article.image}">
<div class="image">
<a href="{article.get_href}"><img src="{VIVVO_URL}thumbnail.php?file={article.get_image}& size=summary_medium" alt="image" /><br /></a>
</div>
</vte:if>
<h2><a href="{article.get_href}"><vte:value select="{article.get_title}" /></a></h2>
<span class="summary"><vte:value select="{article.get_summary}" /></span><vte:if test="{article.body}">...</vte:if>
<div class="article_link">
<vte:if test="!{article.get_link}">
<vte:if test="{article.body}">
<a href="{article.get_href}"> <vte:value select="{LNG_FULL_STORY}" /></a>
</vte:if>
<vte:else>
<a class="visit" href="{article.get_link}"><img src="{VIVVO_THEME}img/external.png" alt="{LNG_VISIT_WEBSITE}"/></a>
</vte:else>
</vte:if>
</div>
</div>
</div>
</vte:template>
Here you need to replace <a href="{article.get_href}"> with <a href="{article.get_link}">.
Now you can add your articles with short summary, and type your URL in "Link" field on the left side in Options type, and that way you'll have short article with external link on title and after short text.
Hope It helps.