Vamos comer = Let's eat - an informal suggestion.
Let us eat is a request for permission.
Please, sir, said Oliver Twist, let us eat something. We are hungry.
How does one distinguish between "We will eat more rice" and "Let's eat more rice"? Not to mention that the imperative hasn't even been taught at this point...
You just get the difference in the context....
Isn't it also "We are going to eat more rice." ? Couldn't it also be "Let's go eat more rice." ? which, of course, was not accepted for me.
Yes, it is also right.