Imperative Vs. Subjunctive

Question

I have a question regarding the following excerpt:

"Both "¿Por qué no...?" and "Mejor no..." followed by El Presente are more frequently used in speech than their equivalent forms using the imperative:

Vayamos a la playa.
Let's go to the beach! (affirmative command)

No vayamos a la playa.
Let's not go to the beach! (negative command)"

My understanding is that whilst 'vayamos' is the negative imperative form of 'ir', 'vamos' is the affirmative form, contrary to the above.

Is there a situation where we would use the subjunctive 'vayamos a la playa' instead of the affirmative imperative 'vamos a la playa' to express the same command?

Many thanks,

Stu

Accepted Answer

Hello there Stu -Yes, in Spanish "vayamos" is regarded as a 'first-person-plural' imperative. This construction is a standard feature of grammar in Romance languages - but it does not occur in English [as Michael Swan points out in "Practical English Usage"]. Thus, the best translation is "let us [go]".Just as Inma implies in this lesson, the choice between different possible expressions is often a matter of 'taste' (although it is true that 'vamos' is used much more than 'vayamos': Butt and Benjamin actually describe 'vamos' as an irregular first-person-plural imperative of the verb "ir").

Answer

Hola Stu
Following David's explanation I'd say that "vamos" is more often used/more colloquially used as an affirmative imperative for ir in the nosotros form. The slightly more formal "vayamos" is the one following the rule on how to form the imperative for "nosotros" as in "let's..." because the rule is to use the present subjunctive form of the verb, in this case, "vayamos" (not vamos). 
So, for that specific sentence you can either say:
¡Vamos a la playa! - more informally
¡Vayamos a la playa! - a bit more formal
¿Por qué no vamos a la playa? (using present indicative vamos)
Saludos
Inma