Everyone in Vietnam seems to be learning English. Standards are relatively high considering the country has only been open for just over a decade. Most young people and many of those working in the tourist industry speak sufficient English to communicate at a basic level. You’ll find more and better English-speakers in the south - a legacy of the American presence - but even here don’t expect to find English spoken at small restaurants, in markets or anywhere off the tourist trail. For such situations it will help to have a basic phrasebook.
People over 60 years old, especially in the north, speak wonderfully old-fashioned French. Other northerners might speak Russian or German, depending where they were sent to be educated or as “guest workers”.
If you’re having real difficulties communicating, it sometimes helps to write things down in English. As a last resort, someone will probably go and find an English speaker to help sort things out.
Though you will certainly be able to get by in English, it’s worth learning a few Vietnamese phrases before you go. The pronunciation is a bit tricky, but otherwise Vietnamese is not a particularly complicated language. A few standard phrases (such as hello, thank you, how much is it? and goodbye) always go down well. It will also help if you learn the numbers, though this can be circumvented by asking people to write down prices, times etc.