In the largest cities, Hanoi and Ho Chi Minh City, tourist infrastructure is good, and hotels and restaurants generally have staff who can communicate in English. It is not difficult to get transportation between hotels and restaurants or tourist destinations like museums and markets.  However, sometimes the most interesting places to visit are outlying neighborhoods and smaller cities and towns which do not get a lot of tourists, and consequently the number of English speakers, as well as their ability with the language, is much less. Since Vietnamese is a tonal language and uses sounds which are totally unfamiliar to the western ear, getting around with a phrase book is not practical. 

Often the agency has guides who will travel outside the city in which they live, and who can make arrangements for a car or van (much safer than trying to navigate unfamiliar streets which often have heavy auto and motorbike traffic.) They can lead excursions in regional cities to see cottage industries or agriculture, or they will know other agencies which can do so. One of the most interesting things to do with a guide is to walk through a local market with all of its unrecognizable foods and have them identified -- including free range rat, just trapped in the rice paddies, ready to take home and cook!

Tourists who are planning to base their travel out of Ho Chi Minh City or Hanoi might want to make advance plans with the agency so that they can have the same guide for the entire time. This lets the guide get to know the client better, and provides the visitor with someone who can interpret the culture as well as the language.