Defoe has been pushed down the pecking order at White Hart Lane following the arrival of fellow striker Roberto Soldado from Valencia, and he is believed to be a target of former Spurs boss Harry Redknapp at QPR.
Iceland international Sigurdsson has also been linked with an exit in the transfer window, after failing to feature regularly in Villas-Boas' side last season.
However, the Tottenham boss insists that neither player is on their way out, with both firmly in his plans for the upcoming campaign.
"They are players we count on 100 per cent," he was quoted as saying by The Daily Express.
"Sigurdsson is completely in our plans. Defoe is a hero for Tottenham and has been doing well. He faces competition which he has always faced during his career."
England international Defoe has been at the club since 2009, while Sigurdsson moved from Hoffenheim in 2012 after a brief loan spell at Swansea.