In a nonreactive bowl or measuring pitcher, whisk the apple cider vinegar, granulated sugar, and kosher salt until the sugar and salt dissolve, 1–2 minutes.
Stir in the crushed red pepper flakes, ground black pepper, and cayenne pepper until evenly distributed.
Funnel the sauce into a clean glass bottle or jar and seal.
Let stand at room temperature for 2 hours to meld, then refrigerate for at least 2 more hours before using. Shake well before each use.
Mop onto pork during the last 30–60 minutes of cooking and toss with pulled pork just before serving. Store refrigerated for up to 3 months.