In lg warmed bowl, sprinkle yeast over water, stir to dissolve and let stand until bubbly. Blend in starter, sugar, salt and vinegar. Gradually beat in 3 c of the flour. Beat at least 3 mins. Turn batter into a lg oiled glass or ceramic bowl, cover with towel and let rise in warm place 1 hr or until double in bulk. Combine 1 c of the remaining flour with baking soda. Stir batter down and add flour-baking soda mixture. Gradually add remaining flour to make a stiff dough. Turn out onto floured board and knead, adding additional flour only as needed to prevent sticking, approx 300 strokes of folding and turning or until dough is smooth and elastic. Sprinkle a greased baking sheet with cornmeal. Form dough into 2 oblong loaves and place on sheet. Cover with towel and let rise in warm place 1 to 1-1/2 hr or until not quite double in size. With sharp razor, slash the tops of loaves diagonally. Mist with water and bake in 450 oven 10 mins. Reduce heat to 400 and bake 35 mins longer or until bread tests done. For a harder crust, place a pan of hot water on bottom of oven and mist with water several times during baking. Remove pan of water after 15 mins of baking. Turn out onto wire rack and cool.
NOTE: If you like your sourdough very dark, remove the baked bread from the pan or sheet and place under broiler about 2 mins, or until rich brown in color.