It's clearly meant for a cable housing end, so I'm guessing it's for the front brake or clutch. Looking at my big Vespa parts book, my best guess is that it's PN 90511, labeled "terminal." When I Google for that, I find a metal piece called a "top hat." It's not split down the side like this is. But maybe someone substituted this at some point.
SIP has this: https://www.sip-scootershop.com/en/products/stop+cable+sleeve++45+mm_17701000
Any other thoughts? I don't have anything like this on either cable right now. Maybe that's why my front brake and clutch are slightly wonky.