After a suspected gas leak threatened to initiate an evacuation from the Oakwell press box and delayed kick-off for 15 minutes on Saturday, Boro were forced into an early defensive reshuffle when Rhys Williams was taken off on a stretcher after going down injured in the opening minute.
The centre back, in his first start in nearly a year, called for treatment after going to ground in what ended as a sad return after such a lengthy time-out through injury.
He was replaced by Milos Veljkovic, who dropped to central midfield with Dean Whitehead switching to right-back.
The visitors were well on top in the first half with Kike, Vossen, Lee Tomlin and Patrick Bamford all going close.
Barnsley’s Conor Hourihane had a shot from the edge of the area which cleared the bar, but the game remained goalless before the first half drew to a close.
Vossen found the target shortly after the restart after pouncing on a loose ball inside the area and slotting a low shot past Ross Turnbull.
Kike threatened to increase Boro’s lead when he headed against the bar from Tomlin’s cross in a classic counter attack from the visitors.
Middlesbrough’s Adam Clayton was then sent off in the 77th minute after receiving a second yellow card for a challenge on Leroy Lita.
Despite being down to 10 men, Boro’s Ayala added a second goal six minutes from time after meeting Grant Leadbitter’s corner with a header.