Having seen Leo Baptistao spurn an early opportunity, the visitors eventually went on to take the lead in the 14th minute.
Lass was the creator, drilling in a cross for captain Piti, who converted from close range.
Goalscoring chances were then in short supply, with neither goalkeeper called into any telling action.
It was Rayo who made the most of their limited openings when they sealed the outcome with seven minutes left to play.
Jose Manuel Casado played in Delibasic and he fired into the net from inside the area.
The result has move Rayo up to eighth in the La Liga standings.