There isn't really anything that needs to be added. Just edit the showthread_quickreply template.
Change:
Code:
for="qr_quickreply"><input type="checkbox" name="quickreply" value="1" id="qr_quickreply" accesskey="w" tabindex="4" />$vbphrase[quote_message_in_reply]</label>
To:
Code:
for="qr_quickreply"><input type="checkbox" name="quickreply" value="1" id="qr_quickreply" accesskey="w" tabindex="4" checked="checked" />$vbphrase[quote_message_in_reply]</label>
what this does is make it to where when you click on the quick reply button, it brings you down to the quick reply box, and automatically quotes the message that you clicked the quick reply button on.