Remove necks and giblets from hens if present; reserve for another use or discard. Rinse hens and pat dry. In a 1-gallon zip-lock plastic bag, mix vinegar, 1 tablespoon olive oil, soy sauce, chopped sage, and half the garlic. Add hens. Seal bag and chill, turning occasionally, at least 1 hour or up to 1 day.
Rinse squash, potato, and bell pepper. Cut off and discard skin from squash. Stem and seed bell pepper. Peel onion. Cut squash, potato, pepper, and onion into 1/2-inch cubes; put in a 9- by 13-inch baking pan. Add remaining 2 tablespoons olive oil, remaining garlic, the whole sage leaves, 1/4 teaspoon salt, and 1/8 teaspoon pepper; mix well.
Lift hens from marinade; discard marinade. Set birds slightly apart, breast up, on a rack in a 10- by 15-inch baking pan. Tie ends of drumsticks loosely together with cotton string.
Place vegetables on the top rack and hens on the bottom rack of a 425° regular or convection oven. Roast, stirring vegetables occasionally, until vegetables are browned and tender when pierced and hens are browned and meat at thigh bone is no longer pink (cut to test), 45 to 60 minutes.
Add salt and pepper to hash to taste. Scoop onto plates or a platter. Arrange hens alongside.