vos

Vos is the plural of tu, and it declines vos, vestrum or vestri, vobis, vos, vobis. The two genitives are not interchangeable. Vestrum is PARTITIVE, used when you are taking some out of the group: unus vestrum, "one of you", nemo vestrum, "none of you". Vestri is OBJECTIVE, used when the pronoun is the object of the idea in the noun or adjective: memor vestri, "mindful of you". Nostrum and nostri split the same way.

With cum the preposition comes after and joins on, vobiscum, exactly as with mecum, tecum, nobiscum and secum. Cum vobis is not written.

Because a Latin verb already carries its person in the ending, putting vos in the nominative at all is emphatic: you and not somebody else. And note that classical Latin has no polite plural for addressing one person. That habit begins in the late empire and it is the ancestor of French vous and Italian voi, and of the whole polite-plural distinction that Latin itself did without.