In small bowl, cover dried shiitake mushrooms with ½ cup (125 mL) warm water; let stand until softened, about 15 minutes.
Meanwhile, slice stems off fresh shiitakes; place stems in saucepan and set caps aside. Add chicken stock, 1 cup (250 mL) water, dry sherry, onion, garlic, bay leaf, peppercorns and thyme to pan. Bring to simmer; cover and simmer for 30 minutes.
Meanwhile, reserving liquid, remove soaked mushrooms and slice off stems; discard stems. Slice soaked and fresh shiitake caps thinly; set aside. Strain stock mixture into clean saucepan. Using sieve lined with coffee filter or triple layer of cheesecloth, strain reserved mushroom soaking liquid into pan.
Add mushrooms and bring to simmer; cover and cook until tender, about 10 minutes. Stir in vinegar. (Make-ahead: Let cool. Refrigerate in airtight container for up to 24 hours; gently reheat to serve.) Ladle into warmed bowls; garnish with chives.