The song was co-penned by Cathy Dennis, the songwriter behind Kylie Minogue's 'Can't Get You Out Of My Head', and Eg White, whose credits include Will Young's 'Leave Right Now' and Adele's 'Chasing Pavements'.
According to a press release from concert promoters SJM, 'Once' is a "quirky, synth-based single" featuring the hook: "I'm only going to let you kill me once."
Vickers's currently-untitled debut album, which features collaborations with Ellie Goulding, Guy Sigsworth and Lightspeed Champion, will follow in late April.
She will preview material from the record on her first headline tour - announced earlier today - which kicks off at Bristol Cooler on March 20.
The 18-year-old singer, who finished fourth on The X Factor in 2008, is currently appearing in the West End production of The Rise And Fall Of Little Voice.