For the vegetable stock: Add the vegetable stock, onion, carrots and celery to a large saucepan. Bring to a boil, then maintain at a simmer while making the risotto.
For the risotto: Add the olive oil to a large heavy-bottomed saucepan set over medium-high heat. When hot add the barley and saute until lightly toasted, 2 to 3 minutes. Add the shallots, garlic, Vidalia onions and salt; cook, stirring often with a wooden spoon, until the shallots become translucent, 6 to 8 minutes. Add the shiitakes and thyme leaves and cook for another 3 to 4 minutes. Next, begin adding the vegetable stock, one ladleful at a time, allowing each ladleful to be absorbed before adding the next. Stir frequently throughout; this brings out the starch in the barley. When the barley is tender and all of the vegetable stock has been used, remove the risotto from the heat and stir in the butter, Parmesan and pepper. Transfer to a serving platter and garnish with the parsley. Serve immediately.