The home side dominated the opening exchanges and went on to break the deadlock in the 18th minute. Gaston Sango was the scorer, slotting the ball into the corner of the net from close range.
Gijon continued to control the match but six minutes after the restart Mikel Labaka drew the away side level with a strike from six yards out.
However, the game swung back in Gijon's favour when goalscorer Labaka was handed his marching orders.
Gijon made their numerical advantage count 12 minutes from time when Mate Bilic struck in the winner from outside the area.
Casado ensured Vallecano ended the encounter with nine men when he was handed his second caution in the last minute.