If you're arriving from the north or from Florence, take the A12 Genoa-Rosignano motorway and continue on the Aurelia Bypass (SS1) towards Rome. You can exit at San Vincenzo North or at San Vincenzo South: both toll gates are well connected to the centre and to the main accommodation facilities.
If, on the other hand, you are coming from the south or from Siena, once you reach the Variante Aurelia (SS1) towards Livorno, you can use the San Vincenzo Sud or San Vincenzo Nord exits.