Who was the first person to put forward the creation of vernacular poetry?

Bai Juyi, the earliest and most famous person who emphasized the oral popularization of poetry, has a famous saying: Write poetry so that an old woman can understand it.

Volume 1 of Hong's Cold Zhai Night Talk reminds me of this famous story: "Every poem (Bai Juyi's word Lotte) is asking,' Is there any solution? "If it is solved, it will be recorded; If you don't understand, it's easy. " What does this sentence mean? It is said that Bai Juyi's self-demand for poetry is "to make an old woman understand"

Yi: Older women. According to legend, every time Bai Juyi wrote a poem, he read it to old women. If he didn't understand it, he changed it so that they could understand it. "Make the old woman understand" describes that poetry is easy to understand.