In tourist areas you'll be fine but if you're off the beaten path then it's going to be harder. It also depends on who you want to communicate with - bartender? They probably speak some English. Street vendor? Depends if they are in a touristy area or if they are in any area less frequented by tourists. Person on the streeet? Maybe, especially if they are younger, but maybe not.

Vietnamese is hard because it's a tonal language and it uses a mostly-Latin writing system but it can be quite deceptive as to how to pronounce words (e.g. the last name Nguyen), so it's better not to assume that you can't say the word just because you can read the letters. Vietnamese people are generally very friendly and accommodating, so you could absolutely ask a person on the street to help you with translating and you've got a good chance of them saying yes. Also they tend to be very patient so it's unlikely that you'll get a rude response by doing things like pointing at a menu and holding up one finger to indicate an order.

Do you have any dietary requirements? I'd recommend learning the words for those if you do. Otherwise a few phrases will be enough to get you by such as "Xin chào" for hello (Sin Jow - as in the word jowl minus the l), "Cảm ơn" or "Cảm ơn nhiều" for thank you/thank you very much ("Gam/Gum ehh-n new"). Note that the letters are recognisable but if you tried to pronounce these words phonetically as a native english speaker it will be utterly incomprehensible to a Vietnamese person. It's not the most English-friendly country in SE Asia that I've ever been to but it's not that hard either.