Vosotros

Who do you need, who do you love, when you come undone?

<<How do you know if you means one person or several ones?>>

Like in many other languages, by context.

<< <<How do you know if you means one person or several ones?>>

Like in many other languages, by context. >>

True.

In those Spanish varieties that lack 'vosotros', in plural, "I saw you" is the same as "I saw them", "Where were you?" = "Where were they?". Everytime I have to think for a moment, because the first thing that comes into my mind is 'they'/'them' instead of 'you'.

Does the use of 'vos' differ in the countries that use it? In other words, are the verb forms used after 'vos' the same in Argentina, Central America, et al.?

Also, is 'vos' used in all the various tenses?

<< Does the use of 'vos' differ in the countries that use it? In other words, are the verb forms used after 'vos' the same in Argentina, Central America, et al.? >>

In Present Tense
1) Argentina, Uruguay, Paraguay, Central America, Mexico (Chiapas), Colombia, and Bolivia have the same conjugation which is pretty easy: Replace the final -R of the infinitive with an -S and add a tilde to the vowel before the -S.
hablar => hablás
pedir => pedís
entender => entendés

The only exceptions are:
ser => sos
ir => vas

2) In Chile, from the infinitive, final -ar is replaced by -ái, whereas final -ir & -er by -ís (pronounced like -íh)
jugar => jugái
venir => venís (veníh)
querer => querís (queríh)

The only exceptions:
ser => soi/erís
ir => vai

3) In Venezuela, the same forms as "vosotros"
pensar => vos pensáis
oir => vos oís
saber => vos sabéis

4) In Ecuador and parts of Bolivia, "vos" is conjugated like "tú"
vos vienes
vos cuentas
vos puedes


<< Also, is 'vos' used in all the various tenses? >>

Yes. Although in Rioplatense, for subjunctive and preterite, conjugation of "tú" is preferred.

In Rioplatense:
"que te diviertas" is more common than "que te divirtás"
"vos llegaste" instead of "vos llegastes"