A bit old but still mostly true: http://www.craftymind.com/2008/04/18/updated-elastic-racetrack-for-flash-9-and-avm2/
So shortly said - ENTER_FRAME is dispatched at the beginning of the frame, then code is executed, and after all that, the render begins (I don't know what you exactly mean by vblank, vsync or 'beginning of the frame on the screen')..