Avocation suggests a hobby, something one does apart from regular work. One's vocation is one's calling, one's principal endeavor or source of livelihood."My vocation is teaching school, but my avocation is gardening." Calling is synonymous with vocation but is usually applied to work that has been entered as a result of a call, summons, or strong impulse: "He began the study of theology as a result of an inner calling." "He felt a calling to become a minister."
Dictionary of problem words and expressions. Harry Shaw. 1975.