Though thousands of years of evolution have seen Asian cultures markedly diverge, there is a continuum of culture, however that thread may be, that runs from border to border, across waterways and mountain ranges, and through the hearts of the various peoples. They say the quickest way to the heart is through the stomach, so why not consider some of the cultural similarities that can be gleaned by considering culinary similarities?
Pho (“Phở”) and udon both have clear, delicate broths that are indicative of the Asian palate, as compared with soups of European ancestry that often feature rich, cream or tomato-based broths. Some vegetables or meats may be added in with the noodles (leafy greens being a commonality), but notable differences arise in composition once you start looking at the noodles themselves, as well as how the soup base is structured.
Pho noodles are a variety of rice vermicelli, while udon noodles are made from wheat flour. Vermicelli is a much lighter, less chewy noodle, and is vaguely translucent as compared to the solid whiteness of an udon noodle. Both noodles are of course delicious, so it just depends on what your mood is. Incidentally, rice vermicelli does exist in Japan as well, where it is known as “haru same” (pronounced haru sahmay), but it does not play as central a part in cuisine there.
Both pho and udon generally have meat-infused broths, but pho’s meat is beef (with some fish sauce often added towards the end), while udon’s broth often garners flavor from dried bonito flakes. Soy sauce, in varying concentrations depending on the region, adds depth to udon’s broth while in Vietnam, the south tends to eat it sweeter and the north eats it with more green onions.
Both pho and udon almost certainly have roots that can be traced back to China, which should not be surprising to seasoned Asia hands. However, regional palates demand regional interpretations, which over time continue to evolve. Even today it is not uncommon to find both pho and udon with new toppings and flavors. Evolution can be delicious!