Thanks again everyone!
We are ticketed for Vaux-le-Vicomte. I am wondering about lunch. Is the on-site restaurant expensive? The site mentions picnics, but I would imagine that is going terribly inconvenient for US tourists to pack and travel with a picnic.
Regarding the train to Vernon, I see the various prices for different train times on the OUI.sncf site. Are those prices always the same? I am not sure if the prices will vary like airline tickets, or if I should worry about specific train times selling out. I would prefer to buy them that day if it is going to be all the same.