New scooter tires can be slippery
You should be especially careful for the first fifty or so miles when riding on your new Vespa tires, as they can be rather slippery until they wear in a bit.
You may think that your new tires, with loads of tread and fresh rubber, will grip the road better than the tires you just replaced. Unfortunately this is not true.
The problem is that tires are made in a steel mold under immense pressure, and anufacturers coat these molds with lubricants. These help release the tire cleanly from the mold, and also give it that attractive new tire sheen. However lubricants are not something you want on your tires, especially when you only have two wheels. So it is critical that you are extra careful when you ride a Vespa with new tires.