V-bomber tankers in a larger Commonwealth air force

Would a UK/Commonwealth superpower have much reason to retain V-bombers as tankers instead of buying VC10-type aircraft?

Probably very little, provided it had bought enough VC10-type tankers to cover the requirement.

Retaining V-bombers as tankers would offer little incentive in the proposed Commonwealth superpower if enough VC10-type replacements had been bought. The qualification is the number of available replacement aircraft. A larger political grouping would still have to purchase enough tankers to cover its needs.