How much to tip Vietnamese pho restaurant servers?
Sounds like some people are out of date. The acceptable standard tip (average service) for servers has been elevated to 17-18% since about 2000 or so. As far as the actual question, tip the same as is appropriate at any other restaurant, and remember or learn about the difference in culture you are dealing with. Vietnamese culture is not attentive in the same way that Americans are used to expecting, and if you feel neglected, you are probably in a very traditional place with very recent immigrants. I see plenty of intentionally neglectful, American servers walk home with 20% of their sales, or better at the end of the night, so I have no problem leaving a standard 18% tip in a Vietnamese noodle house. For really good service I tend to leave around 20-22%, and for truly exceptional service I am likely to leave 25% or better.