I am pretty sure that if you are just writing multiple bytes to memory in sequence you never need to check ready after the first byte. Do some testing but in most of my stuff I don't check when writing more than 1 byte in squence into the VDC ram. It's only when doing new registers or block copy etc you need to check.
As much as I understood so far: Once you have waited for the VDC to say it is ready for access, you have a window of opportunity. You can access VDC ram as long as it's 'revolving door' is open (as Mr. Herd called it)
If you just wait for the ready bit it won't change. The VBlank bit will change. If you write to D600 the ready bit will be 'ok' as soon as you can access the VDC-RAM (meaning it isn't using this itself) You can use the autoincrement to keep writing without checks until VDC reaches RAM access again. I think this means that: - you can always talk to registers (learn from the experts c-128.freeforums.net/thread/67/correct-write-vdc-registers ) - you must make sure you are in a 'VDC-does-not-need-RAM-itself' moment for any RAM access
You can for example wait for Vblank and then you have the whole of the Vblank to talk to VDC RAM. That is why people use interrupts to sync so that they would not need to wait and know how long they have The "how long I have" would appear to be at least a NTSC/PAL difference as well as well a function of settings (Also, I assume the VDC access will be more frequent if you do not use color in VDC.)
I've tried using the REU, someone on the forum said they could get about 1000 (half screen) through (during Vblank presumably)