Heat vegetable oil in a large skillet over medium-high heat. Place peppers in the skillet; cook and stir until peppers are shiny and lightly blistered, 4 to 5 minutes. Remove from heat. Transfer whole hot peppers to 1 quart mason jar and press down.
Combine white vinegar, sugar, garlic, black peppercorns, and red pepper flakes in a saucepan over medium-high heat. Bring to a boil, remove from heat. Pour mixture over peppers into mason jar, leaving about 1/2-inch head space. Screw on 2-piece top; let seal over 24 hours.