Why cannot I use "Vamos, ... " as "Come on, ..."

Question

If the example used "Vamos, ..." as "Come on, ...", why cannot I use it in my Quiz answer? Perhaps the Quiz needs to be more modified to remove the multiple correct options and be more concise.

Accepted Answer

Hola Weiping
You can definitely say "Vamos" for "Come on". Bear in mind that this structure in Spanish using  the gerund for a command doesn't always translate literally in English, but an approximate translation is given. This is the case with the sentence used in that quiz:
"Come on, don't fall behind!"
This could also be translated as "¡Vamos, que os retrasáis!" using the command "vamos" for "come on", but the focus of this lesson is the use of the gerund with this purpose. One approximate translation for come on! here is using "andando" as in saying "walk on!" ( ~ come on).
The questions for this lesson are offering a drop-down menu where you have to focus on which of those four options is the correct one. The only possible one in that case is the one offering the gerund form.
I hope this clarified it.
Saludos
Inma