In a small saucepan, combine the white vinegar, water, garlic, sugar, pickling salt, and black peppercorns. Bring the mixture to a boil over medium heat and stir until the sugar and salt dissolve, about 2-5 minutes.
Remove from heat and let the brine cool for about 5-10 minutes, so it's hot but no longer boiling.
While the brine sits, slice the peppers into 1/4‑inch rings and discard the stems. Pack the peppers snugly into a clean 1‑quart jar (or two pint jars), leaving about 1/2 inch of space at the top. Sprinkle in the red pepper flakes, if using.
Carefully pour the hot brine over the peppers, pressing them down gently so they submerge fully while maintaining about 1/2 inch of headspace.
Wipe the rim, then screw on the lid firmly. Let cool to room temperature, then refrigerate for at least 24 hours before serving. For best flavor, enjoy within 1 month.