Sociedad cement hold on fourth spot
In a meeting between the two revelations of the season, Sociedad's electric attack decided the match at Vallecas Stadium before the quarter-hour mark.
A long goal kick by Claudio Bravo led to left back Alberto de la Bella charging forward to cross for Agirretxe, who slotted in the sixth-minute opener.
Agirretxe then tapped in a cross from Xavi Prieto in the 14th after Vallecano was caught out abandoning its area after defending a corner.
Sociedad rose four points clear of Malaga and Valencia. It has a league-best record of just one loss in its last 21 games.