Final answer:
A polyphonic vocal piece set to a short poem is typically called a madrigal, which is not listed among the provided options. The options given, such as sonnet and ballad, refer to types of poems rather than vocal pieces.
Step-by-step explanation:
A polyphonic vocal piece set to a short poem is called a madrigal. None of the options provided (sonnet, ballad, haiku, limerick) precisely fit this definition. A sonnet is a poem itself, not a vocal piece, consisting of fourteen lines and usually having a volta, or a turn in thought. A ballad is a type of poem or song narrating a story in short stanzas, traditionally using quatrains with alternating lines of iambic tetrameter and iambic trimeter. A haiku is a very short form of Japanese poetry typically characterized by its small number of syllables (5-7-5 syllable count) and focus on nature. A limerick is a humorous poem consisting of five lines. When it comes to the original question, if it's specifically about a vocal piece, 'madrigal' would be the correct term that is not listed in the provided choices.