Just saw this posting.
First of all it is now a bit late in the day but there are certain wedding locations which have an individual authorized by the local municipality to officiate weddings (civil marriages) which are recognized under Italian law. Such individuals could probably officiate at a small service to renew wedding vows.
However, most of such places do this as part of some form of celebration party.
If you envisage a small group present then something might be doable but if it is just the two of you I do not know of anyone who could do just this.
Much of Italy closes down in August and this coming weekend is the first major departure date for Italians going on holiday. Obviously many activities are open in tourist destinations including Tuscany but some may be closed (certain caterers, photographers etc).