In a small pot, bring vinegar, 1/2 cup water, the salt, and sugar to a boil, whisking to dissolve sugar and salt. Remove from heat. Put onion slivers in a glass pint jar or small heatproof bowl and pour in hot pickling liquid. Push onion down to completely submerge. Let cool, then cover and chill 24 hours. Strain.
Make ahead: Keeps up to 3 days, chilled.