So "vulil means short rain season and can mean autumn but if meaning autumn (which doesnt really exist in the tropics) "msimu wa vuli" is better?
Or you could specifiy "vuli la Ulaya" (or explain it if the person should not be familiar with European weather - the first time). I'm usually a bit sceptical to just assume it will be clear when transferring (between continents or cultures).
Could one say miezi ya kumi na kumi na moja too?