Have butcher grind ham and pork together twice.
Combine vinegar, brown sugar, and mustard in a small saucepan; bring to a boil. Boil 1 minute, stirring occasionally. Remove mixture from heat, and set aside.
Soak breadcrumbs in 1 cup milk 5 minutes; add ham mixture, mixing well. Place into a 12-inch cast-iron skillet, and shape into a 12- x 4-inch loaf. Pour vinegar mixture over loaf. Bake loaf, uncovered, at 300° for 1 hour and 20 minutes, basting every 15 minutes with vinegar mixture. Remove loaf to a serving platter. Slice thinly and serve warm or cold.