Vámonos is a very common phrase, but is a degenerate form of the imperative
vayámonos (1st-person plural imperative of
irse, which means
to leave). This is a pronominal verb. The
-nos suffix is a reflexive pronoun.
To form the imperative in this person, the normal
-s ending is dropped before suffixing the pronoun
-nos. A similar thing happens with the 2nd-person plural reflexive pronoun, where the normal
-d ending is dropped before suffixing the pronoun
-os. Have a look at the charts below.
It may be easier to look at a regular verb first.
Here is
levantar in the imperative:
yo | (X) | nosotros,-as | levantemos |
tú | levanta | vosotros,-as | levantad |
él, ella, usted | levante | ellos, ellas, ustedes | levanten |
Here is
levantarse in the imperative:
yo | (X) | nosotros,-as | levantémonos |
tú | levántate | vosotros,-as | levantaos |
él, ella, usted | levántese | ellos, ellas, ustedes | levántense |
Now, here is the irregular verb
ir in the imperative:
yo | (X) | nosotros,-as | vayamos |
tú | vé | vosotros,-as | id |
él, ella, usted | vaya | ellos, ellas, ustedes | vayan |
Here is
irse in the imperative (note the exception to the drop-the-d-in-2nd-person-plural rule for
this verb):
yo | (X) | nosotros,-as | vayámonos |
tú | vete | vosotros,-as | idos |
él, ella, usted | váyase | ellos, ellas, ustedes | váyanse |