Origin of “vos” pronoun

Question

How did the vos personal pronoun come to be? Is it etymologically related to vosotros in any way? Did it develop before or after the other personal pronouns used today (tú, usted, vosotros, etc)? Was it used in Spain and then transferred to Latin America, or did it develop independently in Latin America?

Answer 1

RAE:

Forma de 2.ª persona singular [tú] o plural [vosotros].

So what I gather is that vos is the short version of vosotros when referring to the second person of plural only; however, Vosotros (vos y otros) is strictly for the second person of plural. RAE again:

1. pron. person. Formas de nominativo de 2.ª persona plural en masculino y femenino.

For example:

Vosotros podéis venir a mi casa cuando queráis => You and the others (your siblings, for example) can come to my house whenever you want.

Vos podés venir a mi casa cuando querás => You can come to my house whenever you want. It refers to only one person (tú/usted/you).

Notice how I removed the i from podéis and queráis above. That really varies by country. A Chilean would say vos podéis... vos queráis... however, an Argentinian would say vos podés... vos querás...

This article has a good explanation of above:

A diferencia del pronominal, el voseo verbal no es uniforme para toda América, y se dan tres tipos distintos de voseo: 1) el voseo diptongado, es decir, aquel que conserva las formas cantáis, cantéis, coméis, comáis y partís, partáis; 2) el voseo argentino, es decir, monoptongado en la vocal más abierta del diptongo, y 3) el voseo a la chilena, que conserva diptongadas algunas formas, pero monoptonga otras en la vocal más cerrada, atraído por la tercera.

As far as its origin, the same article linked above says that the use of vos can be traced back to the 4th Century.

En este tema, que, en realidad, es muy amplio, me ceñiré al voseo, o sea, al uso del vos en lugar del tú, que es uno de los fenómenos más característicos del español rioplatense y, también, de los más extendidos y vigentes en casi toda América hispánica.

Hay que remontar el inicio del voseo español al siglo IV de C...

So, to answer your questions:

Is it etymologically related to vosotros in any way?

Yes, vos and vosotros come from the Latin vos.

Did it develop before or after the other personal pronouns used today (tú, usted, vosotros, etc)?

After reading the article I linked, I'd say that it was developed after tú and usted.

Was it used in Spain and then transferred to Latin America, or did it develop independently in Latin America?

Spain, clearly.

Answer 2

According to Wiktionary, vōs is the plural second-person pronoun in Latin. The Latin vōs became the singular second-person vos in Spanish, which was then pluralized by appending the -otros suffix to form vosotros (i.e., vos y otros).

The Spanish tú comes from tū, the singular second-person pronoun in Latin.

A previous question already explained that usted comes from vuestra merced.