Place the diced dried apricots in a small bowl and add the white vinegar. Stir well and push the apricot pieces down so that they are covered by the vinegar. Cover the bowl with plastic wrap and let stand at room temperature for 4 hours, stirring a couple of times.
Near the end of your 4 hours, dice up your onion and peppers.
Add the soaked apricots together with any remaining vinegar in the bowl into a medium saucepan. Add the diced red onion, red pepper and jalapeño pepper to the saucepan. Add the sugar and stir to combine.
Over medium heat, stir the mixture until it becomes very liquid and the sugar has dissolved. Once the sugar has dissolved, increase the heat to medium-high and attach a thermometer. Cook the mixture until it reaches at least 222° F. on your thermometer then remove from heat and carefully pour into a jar and place lid on it. If you don't have a thermometer, you'll want to allow it to cook the jelly at a full, rolling boil for about 2 minutes. A full, rolling boil is a vigorous boil - one that can't be stirred down. Add the liquid pectin (if using) and stir in. Boil for 1 more minute then remove from the heat.
Let the jelly stand in the pot for 5-10 minutes, then spoon into clean jars (or sterilized jars if canning). Allow the jars to cool at room temperature, flipping the jar over every 10 minutes or so, to distribute the solids in the jelly. Once cooled, the jelly must be refrigerated unless the jars have been properly processed in a water bath. If giving un-canned jelly as gifts, be sure to label the jelly to advise the recipient that the jelly must be refrigerated.
Note 1: This jelly will set without added pectin providing the jelly has been heated to at least 221F. If you prefer a firmer jelly or you want to be sure the jelly sets well, you can stir in a bit of liquid pectin at the end of boiling. Be sure to read the notes above this Recipe Card for ingredients notes, step-by-step photos and some great ideas for using your jelly. You'll find a great recipe for using this jelly as a glaze for chicken and salmon, too!