Don't know if AD wants me giving up the trade secret but here it goes. With a vespa the Police don't know how to find the vin, so most of the time they just remove the license plates and the cops bypass instead of having to try and find the vin number. With motorcycles most cops know where to look for the vin so if you take your plate off they will still write you for the vin so it doesn't always work. Plus the cops know it is a shop and are leinent about it with them.
Your argument sounds plausable to me except that you really can't fight the city. The city and suffolk are really difficult to get tickets thrown out (nassau is easy because they will actually bargain with you). You can make the argument to the judge and ad on that you are willing to work out an agreement for a lesser amount and that might get you somewhere but it is doubtful.
eitherway good luck.
BTW I know some great places for parking in mid town, if you e-mail me I'll fill you in.