In a small stockpot, warm vinegar over medium-high heat. After about 5 minutes, when the vinegar reaches 150 degrees on an instant-read thermometer, just before it starts to simmer, add the lemon slices and continue to cook until the lemon peels begin to soften and wilt, about 10 minutes more. Whisk in the black pepper, cayenne, red pepper flakes, and sugar. Continue to cook over medium-high heat until sugar is completely dissolved and sauce reaches 190 degrees, about 10 minutes. Remove from heat and allow to completely cool before using. Remove lemon and refrigerate sauce in an airtight container for up to 8 weeks.