Place vinegar and peppers in a blender; pulse 10-15 times until the peppers are slightly broken down.
Heat pepper mixture, sugar, and salt in a 4-qt. saucepan over high; boil. Lower heat to a simmer and cook, stirring often, for 5 minutes. Add pectin and boil again 1 minute more.
Meanwhile, submerge six 1-cup canning jars, along with their lids and ring bands, in a large pot of boiling water and sterilize over high heat for 10 minutes. Transfer sterilized jars, lids, and bands to a clean dish towel. Fill each jar with hot jelly, leaving at least 1⁄4" of space at the top. Wipe jar rims with a clean dish towel, place lids on jars, and secure ring bands.
Transfer filled jars to a canning rack; place rack in a pot of gently boiling water so that jars are submerged by at least 1"; let boil for 10 minutes. Transfer jars, set at least 1" apart, to a dish towel and let cool, undisturbed, for 24 hours. To test that jars have properly sealed, unscrew bands and lift each jar by the edge of the lid; if the lid holds, the jar is sealed. If it loosens, jar is not fully sealed, and jam should be refrigerated and used within 2 weeks. Sealed jars will keep, in a cool, dark place, for up to a year.