'a' stands for the first letter of the alphabet.
'm' is usually reserved for "minimum", so a was chosen at the same time as 'b' - and the two are thus designated and separated from each other.
Vb — the design speed for maximum gust intensity or the maximum gust penetration speed.
Vb is developed by the designer as a recommended turbulence penetration speed for commercial passenger aircraft rather than using Va, so that higher cruise speeds can be maintained and there is no danger of inadvertent stall. Stalling larger aircraft is not a good idea, not least because of height loss and possible excessive loads in the recovery.