Using a vegetable peeler, peel the grapefruit into large strips. Cut and segment the grapefruits to remove the pith and set them in a separate bowl. Reserve the leftover grapefruit carcasses for juicing later.
Place the peel, turbinado sugar, crushed pink peppercorns, sea salt, and vanilla powder in a large measuring cup/bowl.
Use a muddler or a wooden spoon to muddle the mixture together. Work the sugar mixture into the grapefruit peel until they start to release their oils and become slightly translucent. Add a few of the grapefruit segments (around a half cup) and muddle them as well.
Squeeze the grapefruit 'carcasses' juice into the sugar mixture. Continue to muddle until the sugar dissolves more.
Stir in the rest of the grapefruit segments.
Set aside at room temperature for at least 6 hours or overnight. During this time, the sugar should dissolve, and the grapefruit peel will become mostly translucent.
After resting, add add the apple cider vinegar to the mixture in an jar.
Stir the mixture well or cover and shake the jar until the sugar and salt dissolve completely.
Strain the mixture through a fine-mesh strainer, pressing on the solids to extract all the liquid. Transfer the strained liquid to an airtight container and let it meld for at least 2 days.
Store the Grapefruit Shrub in the refrigerator for a few months and up to 1 year. The acidity from the vinegar and the sugar content will preserve the syrup, keeping it tasting bright and fresh over an extended period.