Combine the vinegar, sugar, salt, allspice, and red pepper flakes in a large nonreactive stockpot, and bring to a boil. Add the cabbage, tomatoes, onions, and bell peppers, and return to a boil, stirring to ensure that all the ingredients have heated through. Preserve Refrigerate: Ladle into bowls or jars. Cool, cover, and refrigerate for up to 3 weeks. Can: Use the boiling-water method. Ladle into clean, hot pint canning jars, covering the relish by 1/4 inch with liquid from the stockpot. Leave 1/4 inch of headspace between the top of the liquid and the lid. Release trapped air. Wipe the rims clean; center lids on the jars and screw on jar bands. Process for 15 minutes. Turn off heat, remove canner lid, and let jars rest in the water for 5 minutes. Remove jars and set aside for 24 hours. Check seals, then store in a cool, dark place for up to 1 year. Makes about 5 pints