Summarize:

Especially when dining with traditional Vietnamese friends or family, you should wait for either the head of the household or the eldest member to eat first. When in doubt, wait to eat until you see one of your adult Vietnamese dining companions also doing so. Depending on your situation, your Vietnamese dinner may or may not begin with a small offering on a family Buddhist altar. Unlike some other kinds of cuisine, Vietnamese food, including pho, is expected to be personalized to your tastes. Even special sauces that have been prepared beforehand can be poured into small sauce dishes and altered with seasonings until it suits your preferences. While eating your pho, you'll naturally gesture as you dine and converse with dinner companions. Avoid pointing at anyone with your index or middle finger, touching someone on the shoulder, or patting others on the back, as all of these are considered rude.  The placement and usage of your feet is also important in Vietnamese culture. Refrain from pointing with your feet or putting your feet on tables. Excessive use of gestures may come across to traditional Vietnamese as rude, especially for women. In Vietnamese culture, when someone feels socially unsure or like they are of lower status, it's common for them to bow frequently and avoid eye contact. This might seem disrespectful to you, but it is actually a sign of respect. Vocalizing disagreement or criticism, especially loudly or publicly, can be viewed as disrespectful. Your comments or suggestions might go over best if saved for a more private time. Though this might be different from you home culture, Vietnamese tend to show deference by seating and serving elders most prominently, then male adults, then other members of the family, like young men, women, and children.
Wait for the host to eat before partaking. Personalize your food without hesitation. Avoid rude gestures. Acclimate to less eye contact. Honor Vietnamese age and gender customs.