Put the cabbage into a 4-cup (1 liter) jar, packing it in as needed. In a saucepan, bring the salt, sugar, water, and vinegar to a boil, stirring to dissolve the solids. Remove from the heat, wait for the bubbling to subside, then pour into the jar. Tuck in the citrus peel. As the cabbage softens, use a spoon to push it down to immerse it all in brine.