Oblak only left Benfica last month for Atletico.
"It wasn't directly Atleti, but a representative who came directly with a clear offer.
"The operation included Oblak, money and the exit of one of our players", said Vieira.
The Portuguese press have reported the unnamed player he is referring to is Nico Gaitán, who Atleti tried to sign this summer but were put off by the €45m price tag.
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