"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.
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"
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.
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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.
" .