Unpredicatable, sudden or eccentric change, notion, caprice, whim or whimsy.
It was the costume she had worn one Sabbath day back in her youth, when she had looked across the meeting-house and her eyes had met young Thomas Merriam's; but nobody knew nor remembered; even young Evelina thought it was simply a vagary of her dead cousin's.
Mary E. Wilkins Freeman's Evelina's Garden. Amazon
Gibson's prose, as always, is portentous, crosscutting tough-guy understatement and poetic vagary.
Tom Leclair, The New York Times, 21 November 1999, New York Times
reviewing William Gibson's All Tomorrow's Parties. Amazon
A principal remaining vagary in the e-ticket area is the matter of buying a ticket for someone else at another address.
In his two practice rounds, Smith has encountered every vagary of Royal St George’s, a shift in the wind that caused a difference of three clubs from one day to the next, beguiling calm and a savage thunderstorm.