The 25-year-old singer and the 'Holding on to Heaven' hitmaker are to take to the stage in the Carling tent at the music event in Chelmsford, Essex, to perform an intimate show after their slots on the main stage.
Foxes - whose real name is Louisa Allen - will be in the tent on Saturday August 16, while Katy B will perform her set on Sunday August 17.
Carling, the official beer and cider partner of the festival, is also giving budding pop stars the opportunity to have their turn in the limelight as there will be a chance to perform with Rockaoke, who first shot to fame on the Chris Moyles Radio One breakfast show.
Meanwhile, Justin Timberlake and The Killers are set to headline at this year's event, while the likes of Ed Sheeran, Bastille, Sam Smith, Tinie Tempah, Blondie and Lily Allen will also be giving performances.
V Festival takes place at Hylands Park, Chelmsford and Weston Park, Staffordshire on 16 and 17 August. For further information visit - http://www.vfestival.com/
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