Not necessarily. I am hitting this question as part of the overall/global practice-button initiated questions after I have completed the entire Portuguese learning trees on DL. As such, I am not "in the section about imperatives", so the exclamation point has to serve as an indication of the imperative nature of this one-word sentence.
In English, there is no higher authority than common usage. It is, in essence, a pidgin language. This can dismay scholars but it is also the source of the vitality of the language. There can be common social situations where "let's" or "shall we?" are perfectly clear from the context perhaps aided by a raised eyebrow or hand gesture. This, not only in Australia but throughout the UK. However, without the context and in written form they are ambiguous.
DaveDfromWV made a comment about a shirt he bought from an airport that had the phrase "Vamos Nessa". Now, I understand the word play and why he bought it, but I'm confused about the message on the shirt. If "Vamos" means "Let's go", and "nessa" often means "in/on that", why does Google translate say that "Vamos Nessa" means "Here we go"? The literal translation would be "Let's go in/on that"?