The OpenVG display is a separate entity from the framebuffer, it isn't drawn onto it. The VC4 composites the framebuffer along with any other graphical layers (OpenVG, OpenGLES, dispman) in realtime when generating the display over HDMI or the official display (and VGA666 I think). The spi based displays just have an ARM side framebuffer. To get the OpenVG window on an spi display you'd need to grab a copy of the OpenVG window with vgReadPixels() and copy it onto the framebuffer you want.
She who travels light — forgot something.