It's optional here. Both "Chuyến bay của bạn khi nào?" and "Chuyến bay của bạn là khi nào?" are accepted.
Vietnamese does not employ subjects, objects, and verbs in the way we are familiar with in English, Spanish, etc. Instead, like some other Southeast Asian languages, it uses descriptive and focal complements that do not grammatically require a verb. Note that while in Vietnamese a verb is often required in order to intelligibly express some particular idea, it's not for the sake of grammatical correctness, as in some other languages.