I'd like to understand this please. Why would the 1M context be kept in VRAM if you're using DSV4 Pro through the API? Or did you refer to different sessions?

Different sessions. With https://github.com/fairydreaming/llama.cpp/tree/dsv4, 1M context with DSV4 Flash takes less than 6GB of VRAM. I can't run DSV4 Pro, but it should take less than 9GB of VRAM for 1M context, based on the numbers shared in https://arxiv.org/html/2606.19348v1.