I have quite similar problem.
On VM 220.127.116.11
I need to put another one picture but in category view (two pictures for one product in category)
The method described above works in product details etc (I tested it), but when I implement it in category view I got error with calling $this, maybe someone did the trick and would be so kind to answer for that
There was one topic about that (I found when I was searching), but somebody made changes in core file components/com_virtuemart/cateogry/view.html.php (and I try to avoid that).
OK, I keep searching, and if I find something, will write.
Thanks in advance.