Is a villi one cell?
No, it is not.
The villi - a plural form - are projections of cells such as in trachea and intestine. For a vilus - a singular form - to be a cell, it has to have genetic material present in a cytoplasm or in a nucleoid in a prokaryotic cell. Since the vilus does not have, it cannot be said that the vilus is a cell. Rather, vilus has blood vessels which vascularize it, lacteal vessel, and a thin layer which encloses the cell.