Yes, we don't have idea if this is singular or plural. So we can say "Você lê" or "Vocês leem", because both is correct. However, we can't say "Vocês lê", because "Vocês" is only used for 3rd person of plural, and "lê" is for 3rd person of singular. Look:
Singular: 1st: Eu leio 2nd: Tu lê 3rd: Ele/Ela/Você lê
Plural 1st: Nós lemos 2nd: Vós ledes 3rd: Eles/Elas/Vocês leem
In English is the same thing say "You read" for singular or plural, but in Portuguese we have this little rules. I hope have helped you!
No, the pronoun is necessary.
Otherwise, that would be understood as a request or an order (imperative) - "Lê!" / "you, read!"
Furthermore, if you said "lê" meaning a present tense action, you would not know who is reading, as it could be "você", "ele" ou "ela". In Portuguese, one would never say just "lê" in that situation. It would be "ele lê", "ele está lendo", "ela está lendo" etc.
'vos' is the 2d-person-plural (m f, direct and indirect) object pronoun: "to/for/of you (all)". It would not be meaningful here.
'vós' (note the accent; accents are meaningful in Portuguese) is the 2d-person-plural subject pronoun, but "has fallen into disuse in Brazil and is only used in a few regions of Portugal. "vocês" has largely replaced it."