Why two cruisers could be worth more than HMS Vanguard

Why might the Royal Navy scrap Vanguard to retain two cruisers?

The same manpower could provide two independently deployable cruiser presences, which was often more useful than one much stronger battleship.

The decision is a force-distribution problem, rather than a claim that cruisers were individually stronger than Vanguard. Similar manpower could crew two cruisers and place useful warships in two locations at once. Those cruisers could defeat other cruisers and handle many routine deployments. Vanguard remained the more powerful single combatant, but the Royal Navy often needed coverage more than one exceptional cruiser-killer. A later analogy calls her the naval equivalent of the Conqueror heavy tank: highly capable in its designed role after smaller, more adaptable systems had become more useful.