Ref the OP - the Congress of Vienna took place in 1814, by which time Britain had been occupying Malta for 15 years. Neither the French nor the knights had anything to do with the situation whatsoever. The Treaty of amiens tried to give Malta back to the knights, but that was a dead letter long before 1814.
There's a story to the effect that when the Congress decided to shift it's attention to disposing of territories outside Europe Lord Castlereagh, the British Foreign Secretary, presented two lists - one of territories Britain was prepared to give back, and one of territories Britain was going to keep. Congress was also told if they thought they could debate what was on each list they could take it up with the Royal Navy...
No prizes for guessing which list Malta was on.