The GPU is pretty powerful in the VF2, you might be able to offload processing to it. I honestly do not know.
The only currently available RISC-V chip I know with vectors is the Allwinner D1 and it is probably a bad choice, because they used a draft for their vectors, so it is only supported by their own binary tool chain.
As for if the VF2 is a good SBC for OpenCV, I guess it depends on the problem space.