I think you're looking at it backwards if you're mapping in map, or confusing remapping mame vs remapping the ipac2. ??
The left side of the list of inputs is mame's internal input name, the the right side is what you press to do that. So you want "UI Cancel" on the left to be mapped to whatever your button sends (joy1button8 or key'V' or whatever).
OTOH, if you're trying to change what the ipac sends, you have to use ipacs mapping application, and change p1b8 to send esc (so you don't need to remap mame).