Can I get by with just English in Vietnam

The short answer is: YES, you absolutely can.
The longer answer is: Yes, you can, but it also depends on where you are in Vietnam.

Fortunately for travelers, most foreigners visiting Vietnam know just a few Vietnamese words such as "Xin Chào" (Hello) and "Cảm Ơn" (Thank you). This basic vocabulary is generally sufficient for a 2 to 3-week trip to Vietnam. English ranks as the second most spoken language in Vietnam following Vietnamese. In urban areas, even if you attempt to converse in Vietnamese, many young locals will respond in English.

However, if you venture away from metropolitan areas and popular tourist destinations, you may find it more challenging to express your thoughts, ideas, and needs. While body language can be helpful, learning a bit more Vietnamese can save you considerable time and frustration. The silver lining is that most Vietnamese people are incredibly friendly. Even if they don't speak English, they will make an effort to assist you in any way they can.