When you live in an urban area there isn't much you can do to fully prevent your scoot from being stolen. All you can do is make it such a pain in the butt for the thief that he passes it up and moves on to the next scoot.
I have seen people rely on their steering column locks. I have also see guys, at shops, that are 5'2 wheel barrow the scooter by lifting the front tire and rolling it into their garage when in neutral, because the owner forgot to leave his/her keys.
P-Series (Vespa, Stella, Bajaj) style scooters weigh approximately 250 lbs. With a pick-up truck and another guy in an alley you could quickly wheel barrow the scoot into the shadows and then the two of you could pick the bike up into the back of the pick-up. Will that ever happen? Probably not, but it could. And for under $150 you could probably avoid that.
My research shows that On Guard is as good quality as Kryptonite and is a little less expensive.
Here's one closer. In order for it to fit, the lock must be facing upward towards the fender. From the looks of it it will be a major pain in the butt to bust up the lock without totally damaging the scooter, which I think thieves would want to avoid.
Below are pictures of how the lock looks installed on a Chetak. Remember a Vespa P or Stella would accept this lock as well.
Additionally -- as a second lock only -- it would be wise to get a long cable clock and run it tight around your legshield and lock that to the Beast lock as well so that the headset could not be turned freely without removing the cable.