This previous answer of mine talks about the history of Vietnamese cuisine in France. More can be said, however, as I did not address specific culinary aspects which would require extra research. I'm quite sure that phở, originally a Northern dish, is a relatively new import (> 2000) in France where Vietnamese immigrants were more likely to be from the South (until recently). Likewise, the emergence of Vietnamese (and, generally, Asian) dishes in mainstream French supermarkets seems to be a recent phenomenon. I may have a look later if I can find the time.