There's a big Viet community in the 'burbs of Boston too, though not as big as in Washington or California. I have to say, I haven't had a bánh mì in the U.S. that tasted like the real deal yet. For one, the bread generally isn't cheap or crappy enough. In fact, part of the problem is that the ingredients are too nice. Also, the flavors are tweaked for non-Viet consumers, which I understand. It's like getting a burger outside of North America: it might look like what you know, but there's something about it that just doesn't quite match up.
Or like when restaurants try to make Philly cheese steak sandwiches with provolone and good beef instead of cheez whiz and thin steak leavins'. Just not the same, and not as good. Although the best American fast food style burger I ever had was in Switzerland. So I'm holding out hope for good banh mi at some point in our corner of the globe.