Was HMS Vanguard already a developed emergency fast-capital-ship design in March 1939?
Short answerNot yet. The March 1939 proposal described a way to assemble a battleship quickly, while design studies and the developed ship followed after war began.
NotesThe answer clarifies the difference between a ship design and a promising concept. The March 1939 idea was to obtain a battleship quickly with available components from the Courageous class and possibly the R class. It had not yet matured into a design study. That distinction limits how rapidly an alternate war could produce a Vanguard-type ship.