Could a preservation law keep HMS Vanguard in reserve?

Would Vanguard survive if legislation prohibited scrapping her until Britain built a new capital ship?

In this counterfactual, yes: retaining her in reserve would let the government avoid paying for a replacement.

A law tying Vanguard's disposal to a new capital ship would create an incentive to retain her. The expected result is museum reserve status. Her survival would leave a separate argument about whether she remained a viable fighting ship. That unresolved question could consume attention in British defence debates.