The 22-year-old Mali international has impressed at the heart of their defence, whilst he also helped his country to third in the recent Africa Cup of Nations.
Premier League outfit Fulham are understood to have posted scouts to watch N'Diaye in action - which has led to speculation about his future.
Whilst Vitoria are likely sellers, they say they have not been approached by any club.
"Formally we have not received any propositions," Vitora sources told Sky Sports.
"It is normal we would look at this at the end of the season."