Hands down the latter. Good M.2 drives can generally get pretty close to the capacity of the bus, and you can fit literally a thousand times more stuff on 4 NVME than you can on any old GPU.
The nvme, by far.
Hard to imagine a reasonable real-world use-case for vramfs. Still cool.
It has been tried in each generation of motherboard design but in an era where GPUs had a custom motherboard slot that normal cards could not occupy it made a sort of sense. And I know there have been times where the northbridge could not saturate as many PCIe devices as one might have motherboard slots. So even leaving the slot intended for GPUs empty or populated with a daughter card might be leaving performance on the table. But I suspect a riser card would fit handily into a 16x slot without blocking more than one or two 2x slots.