Melt 2 tablespoons butter in heavy large skillet over medium-high heat.
Add 1/4 of mushrooms and sprinkle with salt.
Sauté mushrooms until tender and beginning to brown, 3 to 4 minutes.
Transfer mushrooms to medium bowl.
Working in 3 more batches, repeat with 6 tablespoons butter, remaining mushrooms, and salt and pepper.
Bring 7 cups chicken broth to simmer in medium saucepan.
Melt remaining butter with olive oil in heavy large saucepan over medium-low heat.
Add leek, sprinkle with salt, and sauté until tender, 4 to 5 minutes.
Add rice and increase heat to medium.
Stir until translucent, 3 to 4 minutes.
Add white wine and vermouth and stir until liquid is absorbed, about 1 minute.
Continue adding broth and stirring.
Stir in sautéed mushrooms until rice is tender and risotto is creamy.
Stir in 1/4 cup grated Parmesan cheese.
Transfer risotto to serving bowl.