Vanguard used maintained fleet gun stocks, not accidental surplus

Was HMS Vanguard's use of existing 15-inch barrels an unsustainable wartime improvisation?

No. The Royal Navy had deliberately procured and maintained a large replacement pool for the Queen Elizabeth, R, Renown, and Admiral programmes, many of whose intended ships were cancelled or built with fewer guns.

Heavy naval guns were consumable systems whose barrels required replacement over a ship's service life. The Navy therefore ordered far more 15-inch barrels than the number mounted at any one time. Cancelled R-class and Admiral-class ships, plus Renown and Repulse carrying six rather than eight guns, left additional weapons in the managed pool. Vanguard drew on that established inventory, so her battery represented fleet logistics and earlier procurement decisions rather than a few forgotten spare barrels.