Add vinegar, sugar, cumin seeds, celery seeds, garlic and cayenne to a large pot, and bring to a boil over medium-high heat. Reduce to medium-low, and simmer for 5 minutes. Stir in the jalapeño slices, return to a simmer and cook for 4 minutes. Using a slotted spoon, transfer the slices to a clean jar (I used a 28.7 oz weck jar).
Turn the heat up to medium-high and bring the remaining syrup to a rolling boil. Boil hard for 6 minutes. Using a ladle, carefully pour the boiling syrup over the jalapeño slices. The syrup should fully cover the slices (you'll have a little leftover). Insert a clean chopstick or skewer to the bottom of the jar two or three times to release any trapped air pockets. Cover the jar, and let cool completely before storing in the refrigerator. The candied jalapeños can be enjoyed immediately, but they're better if you let them mellow for 1 week to 1 month before eating.
Notes
*Use gloves here if you are sensitive to chiles. Wash your hands thoroughly afterwards as well.