Well in that case the link should look like the following:
PHP Code:
<a href="{vb:link thread, {vb:raw threadinfo}, {vb:raw pageinfo_newpost}}#post{vb:raw lastpostinfo.lastpostid}" class="threadtitle" title="{vb:rawphrase go_first_unread_in_thread_x, {vb:raw lastpostinfo.lastthread}}">{vb:raw lastpostinfo.trimthread}</a> <a href="{vb:link thread, {vb:raw threadinfo}, {vb:raw pageinfo_newpost}}#post{vb:raw lastpostinfo.lastpostid}"><img src="{vb:stylevar imgdir_button}/lastpost-{vb:stylevar right}.png" alt="{vb:rawphrase go_to_last_post}" width="10" height="8" /></a>
Something along those lines. IDK, im not the developer. I do know that lastpostid does need to be registered and called in the PHP file

(Not my forte)