@Trash80toHP_Mini There's three element to the video device booting/usage:
(a) PrimaryInit ; this runs very early (and just once), is very restricted (only SlotManager calls), and only has access to whatever is available in the ROM (i.e. an older version of the SlotManager, not 32-bits QD). Should only be used for basic hardware initialization (such as disabling VBL interrupts or the display itself). It normally fills out the VRAM with 'grey', but that's not really necessary (or blanking), if you don't mind a display showing garbage immediately after power-up.
(b) SecondaryInit ; same as PrimaryInit but this runs after System Patches have been applied, and so has access to the 'proper' functions (newer SlotManager, presence of 32-bits QD).' It's the one that actually should initialize everything for a 'good' device, meaning one that can't really live with just 1 MiB of address space (unless you decide to ignore machines that boots in 24-bits mode, and do everything in PrimaryInit assuming 32-bits mode and a recent ROM in which case you don't need a SecondaryInit). This includes e.g. switching to whichever mode is selected in the PRAM.
(c) Driver ; or really just some helper functions that are called by System/MacOS to use the device (and each function can potentially do multiple things selected by a integer parameter called the 'selector'). This includes getting the base address for the VRAM for the current mode (so that the VRAM can be directly updated by e.g. QuickDraw), changing the CLUT (color look-up table for indexed mode like 2, 4 or 8 bits), switching mode/resolution, etc. Those normally don't touch the VRAM except for some specific selector such as scsGrayPage ("page", if I understand correctly, is basically an area of memory displayable to the screen; you only need one, but if you have more, that means that you can double-/triple-/quadruple-/... buffering).
As far as I can tell, whatever is in the VRAM and displayed on the screen is fully under the device ('s declrom) control until the driver open() function is called, at which point System/MacOS may start writing in the VRAM on its own (and likely the first thing it will do is call the scsGrayPage selector).