Janne, thanks! That's a good find that this Pierre Girardeau was present in Vincennes in 1757.
I added English translations of baptisms for five of Marie Louise Girardeau's children. They all give her husband's name as Joseph Chartier without mentioning Benac. I see on PRDH that the names Benac and Chartier appear together 78 times in Quebec parish records, so that's got to be his name.
Now, to find out more about Pierre! The folks over in Cape Girardeau are researching a Jean Pierre Girardeau who was born December 1, 1723, at Fort Chartres, Kaskaskia, pays des Illinois. He would definitely be the right age to be a godfather in 1757 and a father to Marie Louise Girardeau whose children were born 1775-1785.