The VM plugin has mainly been tested with 1.1.8 and 1.1.9 (the latest). I believe it would work with 1.1.5, knowing that most of the changes between those versions has been bug and security fixes. With that said, we'd highly recommend you upgrade to a newer version of Virtuemart.
Either way, if the VM plugin doesn't work for you, you can easily disable it. We make no modifications to VM itself, so it shouldn't cause any bad effects if you find it doesn't work for you.
Thanks,
Alex
Virtuemart 1.1.5?
