Combine vinegar, water, and salt in a large saucepan and bring to a boil. While the brine heats, prepare 4 pint canning jars. Wash and trim the beans so that they're about 1/2-inch shorter than the jars. Divide the dill seed and garlic among the prepared jars. Pack the beans in the jars, leaving 1/2-inch clearance between the tops of the beans and the rim of the jar. Use a flat plastic spatula to press the beans away from the sides of the jar to fit all the beans in the jars.