Although there are certainly many more Vietnamese restaurants now than before 1975, the adoption was not similar.

Vietnamese restaurants are primarily due to substantial immigration, first with the fall of Saigon in 1975, and then with the loosening of immigration restrictions in the 1980s.

In contrast, French cooking had already been pretty well established in the upper tier hotels of the late 19th century. There was not nearly as large an immigration flux from France after the European wars.

Vietnamese and French restaurants occupy very different niches in the restaurant business. This isn't an entirely scientific approach, but I looked on Yelp for Vietnamese restaurants, both in Boston and San Jose. Yelp indicates a price range from single $ to quadruple $$$$. The majority of Vietnamese restaurants were single dollar sign, a few were $$, and only a two in center of Palo Alto had $$$. In contrast, there were no single $ French restaurants (except for French-Vietnamese) and a few creperies at $$, and dozens in the $$$-$$$$ range.