I wrote, "Where are all of you?" and it was marked wrong. Yes, there's no "omnes" in the Latin model. However, if the speaker insists on using the pronoun vos, otherwise unnecessary, it would seem he's making a point of underlining the fact that the "you" in question is plural. "Where are you all?" / "Where are all of you?"
Flag it, if it's accepted elsewhere, it will be accepted here, otherwise, there are no reasons I think.
All of you, you all, if you talk to someone from a group, and want the person to understand you talk about everyone from the group, not only one person, is valid.
There's no ambiguity in latin with the "vos" of "estis".