Why would Vladivostok not solve the basing problem?

Could the Royal Navy simply use Vladivostok or Weihaiwei as a forward base?

Not without bringing extensive mobile support, because the ports lacked the developed docks and repair facilities needed for major fleet operations.

Neither Weihaiwei nor Vladivostok had enough developed infrastructure to support a major wartime fleet. Weihaiwei possessed little to begin with, while much of Vladivostok's capacity existed in plans rather than in operating facilities. British investment in portable docks and support groups reflected the expectation that a forward force would have to bring its own base with it. In this scenario, American forces might take responsibility for floating dry docks and the associated equipment. A port marked on a map is different from a fleet base able to dock, repair, and sustain capital ships.