Okay, so I looked through the code and I found that viewtopic.php doesn't seem to have a hook that is located above the signature area (but below the posters message). I created my own hook and placed it on line 499/500:
($hook = get_hook('vt_row_msg_after')) ? eval($hook) : null;
Then I modified the manifest.xml file to use that hook rather than the vt_row_pre_display hook (line 684):
<hook id="vt_row_msg_after"><![CDATA[
Now I got it how I want it: http://forums.programming-designs.com/post/5707/#p5707
I'd suggest in the next update for PunBB to add another hook in the place I made mine (line 499/500 of viewtopic.php) as it could be very useful for other extensions too I assume.