I'm going to break from my usual surly tone and be delicate here. Most folks here will have a generally negative attitude about this whole concept and the reason is that post '74 US-market Vespas came an awful, ugly afterthought turn signal system that screwed up the electrics and the design because the US DOT required it to be there. Most owners have removed them to restore the original design of the Vespa and simplify their electrics. Some folks like them and keep em running and some even put them back on to go for a US-style restoration. To each their own.
I'm giving you this background, because any attempt to get info on installing this system will definitely result in a lot of angry Vespa purists giving you crap about drilling holes in your bike and such.
My $.02 would be that if you really must have turn signals for legal or personal reasons where you live, your best option would be to check out motorcycle shops and find an LED solution that can be mounted in some semi-temporary way on your bike. Sometimes people have mounted them to a bracket on the taillight or the luggage rack or something like that. The point being, trying to avoid drilling.
Another solution would be the Bar-End turn signals that came on some european market Vespas and are pretty sweet in my opinion. There's a tutorial on their installation here:
http://www.smallframes.com/
One other point, in my opinion, ANY attempt at turn signals without converting the electrics to 12v will result in extremely feeble lights that are not worth the trouble and in fact lull you into a fall sense of security when using hand signals would be more effective.
good luck in any case.