"Who are those women in the red taxi?"
Translation:Kik azok a nők a piros taxiban?
why is "vannak" not allowed in this instance (i.e. "Kik vannak azok a nők a piros taxiban?" is "incorrect!")
when the "van" is required in "Mi van a televízióban?" for "What is on the television?"
When are "van/vannak" required/optional/incorrect in the Who...? / What...? questions?
If you look at your sentences, you can answer your own question. "What is on the television?" is asking for a subject. It does not have a subject. A similar structure with the taxi would be "Kik vannak a taxiban?". But since the subject (azok a nők - those women) is included, it becomes a "who is this" type of question. And there is no place for "van/vannak" there.
You don't need van when in "X is Y" (X is the subject) why is a noun or an adjective. You do need it , though, if Y is an adverb or something like an adverb. "On the television" conveys an adverbial meaning.