Why HMS Vanguard was not a battlecruiser

Was HMS Vanguard ever officially called a fully armored battlecruiser?

No; she was always a fast battleship, and the phrase arose from contemporary joking wordplay.

HMS Vanguard was conceived and operated as a fast battleship. The alternative label traces to departmental jokes equating a fast battleship with a battlecruiser that had finally received full armor. Repetition detached the phrase from its humorous origin and turned it into a classification myth. This is a concise example of how informal naval language can harden into false technical history.