An 18-inch Vanguard needs a different hull

How viable would an 18-inch Vanguard be compared with the 15-inch design?

It would need a substantially different, larger hull because 18-inch turrets drive beam, length, and performance requirements.

An 18-inch-gun Vanguard would require far more than a gun swap. Wider turrets would force a broader hull, and preserving speed and performance would then require more length and a much larger vessel. Battleship armament choices therefore cascade into hull form and overall displacement.