Anne:
I'm having more doubts now. The smallest size of Serpentiini (the correct spelling this time) is 13 - 15 cm, and this is the only unsigned size. The next smallest size is 34 cm. The base should be perfectly flat with a chamfered edge. Also the spiral doesn't look quite, only three wraps before fading. All that nudges me in the direction of a copy. I'd keep looking, though, as I never fully trust myself. Designlasi.com is a great place to start looking. A later edit: I looked at the examples shown on Designlasi and the Corning Museum of Glass sites and the ribbon of white on them is very close to what is on yours, except that they don't fade before the top of the vase.
David