What mooring Vanguard beside Tower Bridge would require

Could HMS Vanguard fit where HMS Belfast is moored?

Possibly, but only with significant work to the berth, river depth, moorings, and surrounding bank.

Vanguard could probably fit near Belfast's berth, but the dimensions make it a tight physical problem. The mooring position would need to change, and the river might require dredging. Spacing around the hull would also have to be reconsidered. A permanent berth could even require reshaping part of the bank around the ship.