The Welsh winger made a world-record €100 million (£85m) move to the Bernabeu this summer from Tottenham after starring in the Premier League last term.
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"If Bale is worth €100m, then the same goes for Vidal," he told Mediaset.
"Real Madrid expressed an interest this year, but selling him is out of the question so the interest ended there."
Marotta also voiced his praise French midfielder Paul Pogba, who has established himself as an integral part of Antonio Conte's side since moving from Manchester United a year ago.
"We can say the same about Pogba [as Vidal]," Marotta added.
"He's a solid element of the team and we've turned down all the offers we've received for him.
"He's exploded in an almost unexpected way, so compliments to the lad but also to (Antonio) Conte who has managed to get the best out of him.
"If he confirms his status at the highest of levels, as we obviously hope he will, then this should be recognised with an increased salary."