The Mexican forward brought his tally to 13 goals across all competitions with the third goal in a 3-1 win against Racing Santander.
Sociedad built up play on the left side when a cross came into the box and was left for Vela at the far side. He was unmarked and made the most of his opportunity, smashing a left-footed shot past Santander goalkeeper Mario Fernandez.
It's the second goal Vela has scored in the Copa del Rey this season and his 13th overall as the Mexican continues to enjoy a superb run of form that saw him named La Liga's best player for the month of December.
Vela and Sociedad face Elche in La Liga on Monday before Thursday's trip to Santander for the second leg of the quarterfinal.