Though the temps can be a bit brisk - Tuscany is definitely open for business in December. Just remember to be aware of the holidays - when some things will close.
Though many vineyards will be on the very tail end of their production - many will still be open for wine tastings. However, in many cases they will only be open upon appointment so ALWAYS be sure to contact and reserve before you go - especially if you really want to visit the vineyard. Or you could take an organized tour which will not only give you time to enoìjoy the scenery - but they know already who is open (and who isn't).