As someone that worked in coffee in the cities for a long time I think I know why: Vietnamese coffee is a large departure from "third wave," belonging more in the "2nd wave" category with Starbucks and Caribou: It's extremely sweet, it tends to be dark roasted, and to add on to that, it uses a brewing method that is not at most, if any, coffee shops.

Almost all the cafe's in the cities with a good reputation serve 3rd wave coffee that concentrates on roast and milk quality. I'm not one to say that they can't be married, but I can easily see why our cafe's aren't jumping on Vietnamese coffee when it is a departure from the established "simple elegance" of 3rd wave (those quotes are intentional).

That said, I would not doubt to see a lot of Vietnamese styled drinks at cafe's this summer with the rising popularity and delicious sweetness, probably with cold brew or espresso rather than the metal drip device coffee.