Actually I tinkered around with this a little bit and was able to edit the template (SHOWTHREAD) and was able to get this to work.
Changed this code:
PHP Code:
<vb:if condition="$show['largereplybutton']">
<a href="{vb:raw $vboptions.vbforum_url}{vb:if "$vboptions['vbforum_url']", '/', ''}newreply.php?{vb:raw session.sessionurl}p={vb:raw LASTPOSTID}&noquote=1" class="newcontent_textcontrol" id="newreplylink_top"><vb:if condition="$show['closethread']"><span>+</span> {vb:rawphrase reply_to_thread}<vb:else />{vb:rawphrase closed_thread}</vb:if></a>
<img style="display:none" id="progress_newreplylink_top" src="{vb:stylevar imgdir_misc}/progress.gif" alt="" />
</vb:if>
to this code:
PHP Code:
<vb:if condition="$show['largereplybutton']">
<vb:if condition="$show['guest']">
<a href="/register.php" class="newcontent_textcontrol" id="newreplylink_top"><vb:if condition="$show['closethread']"><span>+</span> {vb:rawphrase reply_to_thread}<vb:else />{vb:rawphrase closed_thread}</vb:if></a><img style="display:none" id="progress_newreplylink_top" src="{vb:stylevar imgdir_misc}/progress.gif" alt="" /><vb:else />
<a href="{vb:raw $vboptions.vbforum_url}{vb:if "$vboptions['vbforum_url']", '/', ''}newreply.php?{vb:raw session.sessionurl}p={vb:raw LASTPOSTID}&noquote=1" class="newcontent_textcontrol" id="newreplylink_top"><vb:if condition="$show['closethread']"><span>+</span> {vb:rawphrase reply_to_thread}<vb:else />{vb:rawphrase closed_thread}</vb:if></a>
<img style="display:none" id="progress_newreplylink_top" src="{vb:stylevar imgdir_misc}/progress.gif" alt="" />
</vb:if></vb:if>
This changes the button to always go straight to the registration page for guests when they click on the reply button.