Hi Ann;
I am glad that you brought this topic up. i visited the Chateau de Vincennes back in 1995 and did not meet any American tourists there. It seems to be off the beaten path for Americans. This is a beautiful fort that has a chapel that is a mini-version of St. Chappelle (?); great stained glass window. As you explore the fort you can imagine battles taking place there. As i recall the chateau has a spiral stair case similar to the one at Notre Dame. On the day i visited a television commercial was being filmed and so there were "knights' walking around the battlements. There was a free tour but it was conducted in French. However they lend a booklet that explains everything in English. When i visited there was a mass of school kids - junior high school age - there who seemed to enjoy the experience.
One of the better things that i saw in Paris was at the chateau. Most of the school age kids where white but there was a small number of black kids (folks, forgive me if i am no using the PC terms to describe race). one of them was a girl using a pair of crutches who had mobility problems. The white girls went out of their way to ensure that she had a good experience.
The chateau is surrounded by a grass filled moat. This is where Mata Hari was [INVALID]d.
Another thing that surprised me was to see Chinese from the mainland visiting paris. At that time i though the red government kept a tight lid on who could leave the country.
As i recall you take an east-west metro to the end of the line (east) then walk a few blocks to the chateau. It is probably a half day visit. Very enjoyable for me. There were military archeologists working there so maybe more has been open to the public since I visited.
Enjoy your trip