Marvel at the size of Vannes Cathedral. Schedule in a mass if you can or just wander around and enjoy a break from the outside world.
You're spoiled for sightseeing choice when spending time in Old Town. Vannes Cathedral is a great place to start here, but there are many other fine attractions to consider. Le Chorus Exhibition Centre, Musee de La Cohue, and Vannes Aquarium are near the principal church and and will cap off your day nicely.