Cold Weather Ideas:
Obviously you can bundle up to deal with colder weather. Many jackets come with liners you can add that help. You could wear snowboarding pants over your jeans, but it’s better to just get some decent motorcycle pants.
If you don’t like the idea of wearing stuff, (EDIT: If you don’t like that idea …) you could instead try a lap apron. A lap apron attaches to the rider, so it’s also known as a scooter skirt.
Best options are Scooter Skirts or Corazzo. (http://scooterskirts.com/index.html and http://corazzo.net/?q=node/191 respectively.) If you want something that attaches to the scooter, then try the Tucano Urbano.
Buy it here: http://www.scooteremporium.com/displayproduct.php?product_id=1950&category_id=269 Or get it direct from the Italian site: http://www.wheelup.it/
On the top right click on the “English version”, then look on the left menu for Tucano Urbano, then choose “Legs Covers.” If you want to save some money, try organizing or getting in on a group buy, although it will take some work. (http://www.modernvespa.com/forum/topic16490.html)