Can I get by with just English in Vietnam?
The short answer is: YES, you totally can.
The not-so-short answer is: Yes, you can, but it also depends on where you go in Vietnam.
To your relief, most foreigners visiting Vietnam know barely any Vietnamese words but "Xin Chao - Hello" and "Cam On - Thank you". That is quite enough vocabulary for a 2 or 3 week trip to Vietnam. English is the most popular language in Vietnam after Vietnamese, and in the cities, even when you try to communicate in Vietnamese, the local younger people will just respond to you in English.
If you plan to take a break from the metropolitan areas and all the touristy places, you will have a harder time communicating your thoughts, ideas and needs. Body languages still work but a little bit more Vietnamese will save you tons of time and frustration. The good news is that, most Vietnamese people are very friendly and even when they do not know a word of English, they will try to help you out in all circumstances.