Combine vinegar, soy and sage in a small bowl. Place mushrooms caps, gill-side up, in a large shallow dish and pour the vinegar mixture into them. Let them marinate for about 10 minutes.
Preheat oven 250 F.
Place breadcrumbs on a large plate. Make sure the mushrooms are coated on both sides with marinate; dredge them in the breadcrumbs.
Heat 1 tablespoon of oil in a large non-stick skillet over medium heat. Add 4 mushrooms, gill-side down. Place a heavy, heatproof plate on top and cook until breadcrumbs are crispy, pressing on the plate periodically to flatten the mushrooms, about 4 minutes. Remove the plate; add 1 tablespoon of oil to the pan and turn the mushrooms over. Replace the plate and cook. Pressing once or twice until the breadcrumbs are golden brown. and the mushroom are tender about 4 minutes more. Transfer to a baking sheet and keep warm in the oven. Wipe out the pan and repeat with the remaining 4 mushrooms.