What is it with people from the US/Canada/Australia always claiming that the best Vietnamese food in the world is to be found in their country, that the Vietnamese food in their country is A-U-T-H-E-N-T-I-C, and that the Vietnamese food in France is both inauthentic and bad ? I am not saying that the Viet food in France is better than the Viet food in those countries, but I don't think it is inferior either. My roomate is a couchsurfer so we often have guests from these countries. As I am half Vietnamese, they often talk about Vietnamese food with me, and most of them have this prejudice against Vietnamese food in Paris, even though they have not even tried it. But you know, as they can have the best Vietnamese food in the world in their own country, why bother ?? This always makes me laugh. Some of them even explain me what is A-U-T-H-E-N-T-I-C Vietnamese or not ! Once, a guy told me that the Hoisin sauce and the greens were compulsory with A-U-T-H-E-N-T-I-C pho ... well obviously this guy has never had pho in Hanoi. But why would he go in Vietnam to have pho when he can have the best pho ever in California ? haha
To stay on the subject : as aliettedb said, banh mi just means sandwich so the only compulsory ingredient is the bread. Banh mi does not need pate or maggi or whatever so-called "authentic" ingredient to be called banh mi !
Maggi is very common in overseas Vietnamese kitchens. All the Vietnamese families that I know here in France have a Maggi bottle in their kitchen. It's often used on white rice, eggs au plat (o pla) and in banh mi. I lived in Hanoi for 2 years and I don't remember seeing a lot of Maggi there ... but they probably use a similar sauce from another brand.
Regarding your assumptions about the Vietnamese food in Paris and the local tastes, I think you should explore a bit more because I just had a banh mi with mayo + maggi (!!) in Belleville, and the clientele was both Asian and French.