Can you prepare the green vichysoisse the day before and chill it and then reheat it? Will it lose its vivid grren color?

Not sure, but there's probably some oxidation and thus reduction in brightness of the green color.
If you want to prepare this dish ahead, I would cook (or just undercook) the crust and fish, make the ingredients for the sauce and refrigerate.
Then, day you want to serve, blend the sauce, quickly reheat or finish cooking the fish and serve.