In a small saucepan, combine vinegar and 120ml (1/2 cup) of water. Season with salt and pepper and set over medium heat. Bring to a gentle simmer.
Add the jalapeño slices to a small, heatproof jar. Pour the hot vinegar mixture over the jalapeños. Seal the jar and shake to distribute the vinegar. Allow to cool to room temperature before refrigerating indefinitely.