Technically we left in January 2020 (but only went into the transition period so I get what you mean).It wouldn't have been, we ordered our vaccine supply while being in the EU.
But even still - all the EU nations could have done the same thing as Britain, they decided to opt into the EU procurement. France, Germany could have all went at it alone but decided not to. I genuinely believe the UK would have always went it alone in this.