The president of the Lisbon-based club insisted that the Belgian midfielder is a vital part of the team and stated that he will hold out for the fee written in his buy-out clause
The Belgium international has been linked with a transfer to AC Milan after impressing for Benfica in his first season at the club following a €7m transfer from Standard Liege last summer.
Earlier in the month, Benfica sporting director Rui Costa stated that no offer had arrived for Witsel, and the club's president has now intimated that he plans to honour the €40m figure included in the player's release clause.
"Every club knows what his release clause is," Vieira told Antena 1. "If they come here and pay that fee, then there is nothing that we can do.
"We are very happy with Witsel and we do not think about selling him."
Witsel, 23, made 29 league appearances for Benfica in 2011-12, scoring one goal in the process.