Is vannak necessary here?
Yes, it is. It's left out only when the structure is:
noun = noun: Ezek a férfiak szakácsok.
pronoun = noun (or the other way around): Ők szakácsok.
pronoun/noun = adjective: A szakácsok fiatalok.
When adjectives go to plural? When not?
Music artists who sing at concerts post coVideo...the #NationalHymns in every language...
https://youtu.be/FHNLdHe8uxY
Should "Hol a fiatal szakácsok vannak?" be accepted?
No. Wrong word order.
Keep hol+vannak next to each other.