Welcome to Just Answer and thanks for your question. There is no question that kitchens today are going stainless steel. It is expected in even modest priced homes. Since your refrigerator is stainless and presumably you are buying a new stainless stove, I would have the vent match the stove. If the stove is black, then you can choose between black or introduce copper in the vent. It's really all a matter of choce.
Yes, even though I love the look of copper, in your case I would go with the stainless for the vent, and also the oven. Black trim on both or either would look fine. I'm afraid that introducing another metal would be too busy. I love the cream, black, and red combination. Very smart!