Lake Vico is easiest to reach by car. From Rome, take the A1 autostrada north to the Attigliano exit, then follow signs toward Viterbo and then Caprarola — allow about 1 hour 15 minutes from central Rome. The approach road around the lake starts from Caprarola or Ronciglione, both of which have parking.
By public transport, take a regional train from Roma Ostiense or Termini toward Viterbo (the Roma-Nord line, about 1.5 hours to Ronciglione or Caprarola stops). Local buses connect the train stations to the lake, but schedules are infrequent and a taxi from the station may be more practical. The area is not well served by public transport and hiring a car in Rome is strongly recommended for a day visit.