The singer, who finished fourth in last year's competition, will star as a painfully shy girl with an amazing voice and the ability to impersonate legendary singers such as Judy Garland and Dame Shirley Bassey.
In a statement, the show's producer Nica Burns admitted that she was persuaded to audition Vickers after playwright Jim Cartwright insisted that she was "the perfect girl" to take on the role that made Jane Horrocks famous.
"When I saw Diana I knew immediately he was right. She has a special quality. When she finally agreed - after much persuasion - to come in and read for us, we found out that not only can she sing but she was an intuitive actress as well," she said.
Cartwright added: "This young girl is a natural actress. The moment she read the part, it was magic. A frisson ran through the room. Diana Vickers is the real deal. She is Little Voice."
Burns also revealed that Vickers had been given permission to appear in the production by her record label, after it was agreed that she could take a break from recording her debut album.
Little Voice will open at the Vaudeville Theatre, London on October 20.