Latin has its roots in dozens upon dozens of languages. Learning Latin enables you to grasp the concept of the English vocabulary, Spanish vocabulary, Italian vocabulary, French vocabulary, German vocabulary etc. much more simply than before. 

For example, the word "recapitulate." It comes from the latin stems re meaning again, and caput, meaning the head, or top. It literally means "again from the top." Or, more simply, it's the extended form of recap.

Infidel comes from in, meaning not, and fidel, meaning faith. Not of the same faith.

Doctor, from doctum, meaning teach. Disciple, from discipulus, meaning student. Convert, from con (with) and vert (turn). Conscience, con and scientia, meaning knowledge. Ambidextrous, from ambi meaning both, and dexter meaning right. 

The list goes on and on.

Learning to translate from Latin to Dutch it was always helpfull when a Latin word looked like something in English. 

Now I think learning Latin (and Greek) was pretty useless, but it supposedly looked good on resume. 

English is a germanic language. It's no more "eolved" than Spanish. Spanish evolved from Latin, English didn't.

Oh, dear, it's been abou twenty years since I studied latin... I'll throw in a few cents, but they might be very rusty ones.

Anyway I' d say that the "who" is clearly a relative pronoun, with an ellipsis

"Do they know ['the person] who had been taken by the common people"

And as such I'd go with your first:

"Sciuntne [eum] qui ab vulgo captus erat"

As in latin you also usually ellipsize other pronouns such as this demonstrative one in front of a relative.

As you pointed out, english "who" is ambiguous whe it comes to sex and number of the antecedent, but I'm ignorant of what the latin usage was for that,..

Wouldn't the interrogative pronoun work better in place of "the person" as they don't know whom?

Sciuntne quem qui ab vulgo captus erat.

But "quem qui" sounds stupid.

Gracias.

Again, I'm very rusty, but I'd say that you use the interrogative pronoun when you're interrogating for the identity of the antecedent. As in

"Cuius liber est hic?"

In your case the interrogative is about the truth of "them" knowing about the identity of someone. You're asking "do they know [the person]" (doubt about knowledge)  and not "whom do they know" (doubt about the object of knowledge). The fact that [the person] is actually the antecedent in a different (relative subordinate?) sentence is tangential, I think.

PS: Note that I might have mis-used terms regarding grammatical and sentence analysis commonly used in English, as I'm translating quite freely from Italian and I don't really master the appropriate lexicon. I hope you'll understand anyway :)