You see, vacuum is created by eliminating/sucking out air and if there is no air, then there is also no oxygen and when there is no oxygen, the flame can't burn and dies. Additionally, the front wall only melting adds humidity and coolness to the air outside the wall, thus weakening the fire even before it dies out at the vacuum.
Hitsugaya is not that awesome, since as mentioned, it is a "combination attack/technique", so basically he is unable to create it by himself. The combination is similar to creating window glass.
Basically, Matsumoto creates (multiple) Ash Walls, then Hitsugaya freezes them and then Matsumoto draws back her Ash, which also suck away air in it, creating a vacuum (the holes through the ash comes back are probably closed then by Hitsugaya again).
They used Science to outsmart a Quincy, right after the chapter in which Kurotsuchi said that they will showcase their intelligence :p.