Put the jalapenos and red onions in a 16-ounce wide-mouth mason jar.
In a small saucepan, combine the vinegar, sugar, salt and 1 cup water. Bring to a boil, stirring until the sugar and salt are dissolved.
Pour the hot liquid over the veggies in the jar, screw on the lid and set aside. When it has cooled to room temperature, transfer the jar to the fridge to chill for a minimum of an hour, which will allow the vegetables to pickle. The pickles will last in the fridge for 3 days. Serve them on pulled pork sandwiches.