I created my own hid2vpad ini and just assumed it would then work in retroarch, that's what I was led to believe elsewhere - that is, it would then just "show up" as a choice of devices against a port (5-16, as you said).
You're saying I still need to create some sort of cfg files for retroarch also? It's just a usb joypad plugged direct to the front usb.
To add to this, a dual shock 3 works with no faffing about at all, it seems built into retroarch to me (it works with no controller folder even present). It feels like my retroarch just isn't recognising any hid2vpad ini files at all?