In Vietnamese with diacritic = Phở. In Vietnamese, they are several different types of "o"---o, ô, and ơ. The one in phở, ơ, is pronounced uh. So the above people are correct, it's pronounced "fuh". But that's not it. In Vietnamese, they are also tones. See that question mark thing above the ở? That means you say phở like you're asking someone a question. So when you say it, your voice falls and then rises back up (if you're speaking quickly, it sometimes just falls). These tones are important. For example "má" and "ma" mean two different things even though they are pronounced same because the tone is different (mother and ghost, respectively).
It has always amazed me how the mispronunciation, "foe", has caught on. In Vietnamese, that would be written phô. At the same time, I'm sure many Americans would be shocked at how badly Vietnamese people butcher English.
Source: Being Vietnamese