The date was dropped in an interview with GameSpot Japan. According to Inaba, the demo will be called "Velocity Attack" and will feature a prologue sequence aswell as a boss fight with Argos — no, not the catalogue based retail store.
According to Inaba, the demo will differ from the one being shown at TGS next month — that will apparently feature an appearance from a surprising character. Bayonetta's the name hot on everyone's lips right now.
Vanquish is due in October. We'll be checking if the demo will be coming to the West.