lst = DictionaryLookup[{"Spanish", 
      x__ /; StringCount[x, Alternatives["a", "e", "i", "o", "u"]] == 5}] 

    Short @ lst
> {ababillarse, abadiado, abadiato, abajadero, <15405>>, zurrapienta, zurrapiento, zurriagazo}

    Length @ lst
> 15412