i have never nodded or acknowledged scooter or vespa riders and probably never will. could be because when i started riding there were a lot of guys going down the Mod route, they were usually a bunch of *wats
scooters and vespa's are "normally" ridden by some spotty oik or a parka clad imbecile neither of which seem to exhibit any awareness of anything around them or any roadcraft at all and what really erks me is when the media normally report a "motorcyle" accident you can be sure they mean a 16 year old on a scooter
apart from that i tend to nod to any other motorcycle and that includes harley's to which i do get some nods back i also acknowledge m/c cops who sometimes recipricate
but then i don't nod to get a response i do it because i am a very nice guy........................