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.
Betsy Wade, The New York Times, 19 November 2000. New York Times
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.
Neil Harman, Times Online, 17 July 2003. Times Online
And then he would draw back in fine shades of grey or, as in the lyrical D flat major variation, side-stepping into meditative vagary with easeful rubato.
Garrick Ohlsson, The Times, 9 December 2002. The Times