England batsman Ravi Bopara needed three runs to win the Super Eights match in Antigua but fast bowler Fernando pulled out of his delivery before resuming his run, and clean bowling the batsman.
"A bowler has the right to test a batsman at any time," Vaas explained to the BBC's Sinhala Service.
"I think, as an experienced bowler, Dilhara deliberately did it."
Vaas said Dilhara Fernando wanted to test whether Bopara would alter his stance at the moment of delivery.
"It was very important for the bowler to understand the batsman's reaction. I think it was a very wise move by Dilhara as a senior bowler."
Sri Lanka face New Zealand in Grenada on Thursday in their next match as they attempt to make sure of their semi-final berth.
Vaas said: "Our batsmen and bowlers are in form. If you consider the way we've been playing so far, I think we have a chance to beat New Zealand."