Why is "vamos" used for command but not the imperative "vayamos"? [duplicate]

Question

I don't understand why people use vamos (present) to say "let's go", but not "vayamos" (imperative/subj. present).

Is there a special reason to use "vamos" (vámonos) instead of "vayamos" (vayámonos) to say "let's go" (let's leave)?

Answer 1

First of all, it appears as if the RAE does not include first person plural in the conjugation of the imperative (see conjugation of ir, click on the blue button "conjugar"), so when we are using vamos it is not actually an imperative form, but the subjunctive used with an exhortative value.

So the question is, why use the imperative form vamos instead of the correct subjunctive form vayamos?

Well, according to the Diccionario Panhispánico de dudas, ir, point 2:

The form vamos is, nowadays, the first person plural of the present indicative. [...] However, in medieval and classical Spanish it was, as well as vayamos, the form of the first person plural of the present subjunctive. [...]

As a vestige of its former meaning, the form vamos is still used with an exhortative aim more often than vayamos. [...] As a matter of fact, the form vayamos, with that meaning, has been almost relegated to mere literary discourse. [...]

TL;DR: To sum up, in medieval Spanish both vamos and vayamos were valid forms for the subjunctive, but only the former has been kept in modern Spanish for exhortative sentences.

Answer 2

This passage from DPD explains why "vamos" is used as an alternative of "vayamos" for imperative, first person plural:

2. La forma vamos es hoy la primera persona del plural del presente de indicativo: «Laureano y yo nos vamos al jardín» (Gallego Adelaida [Esp. 1990]); pero en el español medieval y clásico era, alternando con vayamos, forma de primera persona del plural del presente de subjuntivo: «Si vos queréys que vamos juntos, pongámoslo, luego, por obra» (Daza Antojos [Esp. 1623]). Como resto de su antiguo valor de subjuntivo, la forma vamos se emplea, con más frecuencia que vayamos, con finalidad exhortativa: «Vamos, Johnny, vamos a casa que es tarde» (Cortázar Reunión [Arg. 1983]); la forma de subjuntivo vayamos, con este sentido, ha quedado casi relegada a la lengua literaria: «Vayámonos de aquí» (Amestoy Durango [Esp. 1989]). Lo que no debe hacerse en ningún caso es emplear hoy la forma vamos, en lugar de vayamos, en contextos que exigen subjuntivo y sin que exista, en el enunciado, intención exhortativa: «Tenemos una excelente relación [...]. Pero no es que vamos juntos para el cine» (Universal [Ven.] 3.9.96); debió decirse no es que vayamos.

The main reason is that it is a vestige of some medieval use. It is worth noting that, while "vamos" is validly used for imperative, it is wrong to use it as subjunctive.

• Dale, vamos juntos al cine (= Dale, vayamos juntos al cine): CORRECT

• No es que vamos juntos para el cine (better: No es que vayamos juntos al cine)