The San Sebastian-based club were surprise fourth-placed finishers in June and took the lead when Gonzalo Castro's first-time pass sent Mexico striker Vela through the middle to score with a dink over the keeper in the 42nd minute.
As Getafe chased the game, former Arsenal player Vela set new signing Seferovic scampering clear down the left and the Switzerland striker also chipped the ball over Miguel Angel Moya to make it 2-0 from a tight angle in the 70th minute.
Vela should have had his second in the closing minutes but he miscued his shot when one-on-one with Moya.
It was a winning start for new Sociedad coach Jagoba Arrasate who replaced Philippe Montanier at the end of last season, and a morale boost ahead of Tuesday's Champions League playoff first leg at Olympique Lyon.
La Liga champions Barcelona start out at home to Levante on Sunday when Real Madrid also host Real Betis at the Bernabeu.
(Reporting by Mark Elkington editing by Tony Jimenez)
