There's a difference between vacuum bagging and vacuum infusion.
Vacuum bagging probably doesn't save any weight, as the lay up schedule probably remains the same.
Vacuum infusion is a process where the resin is drawn through the glass layup, and requires advanced planning on resin paths to work effectively. Because the resin ratio is calculated to a very tight tolerance this can result in some weight savings.
On a boat the size of a C-Dory, weight savings would probably be in the few hundred pound range, so not real significant. On a larger boat, with a lot of fiberglass pieces, weight savings can be in the few thousand pound range (thing Nordic Tug) whihc is why manufacturers are looking at, and adopting the process. It does add expense to the production of the boat.