Mix together warm water, sour cream, and vinegar in a small bowl. Set aside.
In a large mixing bowl combine 1 cup flour, sugar, salt, and yeast. Add sour cream mixture to flour mixture. Blend together on low speed until moistened. Continue to blend on medium speed for 3-4 minutes.
By hand stir in 2-3 more cups of flour until dough pulls cleanly away from bowl.
On a well-floured surface knead in additional 1-2 cups of flour until dough is smooth and elastic. Place dough ball in a greased bowl, cover with plastic wrap and a clean dish towel. Let rise in a warm (80-85 degree) place until doubled. Approx. 30 minutes.
Grease a large sheet pan. Punch down dough and divide in half. Roll each half into a 14x8 rectangle. Roll, starting from a long side. Pinch seam and ends closed. Place seam side down on baking sheet. I have also shaped these loaves into rounds, typical of sourdough bread.
With a sharp knife make 4-5 1/4 inch deep diagonal slashes in the top of each loaf. Cover with clean dish towel and let rise in warm place until doubled again. (Approx. 20 min.)
Heat oven to 375*. Bake 30-35 minutes until golden and hollow sounding when tapped. Remove loaves immediately to a wire rack to cool.
**NOTE: I often make this dough in my bread machine. Add ingredients in the order prescribed by your machine manufacturer and use the dough setting. I usually add 4 cups of flour to start. After the first rise, knead in any additional flour to obtain the standard elastic dough. Rest dough five minutes before rolling out and continue at Step 5.