In a small saucepan, warm the milk with the shortening over low heat until the shortening is almost melted. In a large bowl, stir the yeast into the warm water, sprinkle with a pinch of sugar and let stand until foamy, about 3 minutes. Stir the warm milk mixture into the yeast along with the 1/2 cup of sugar and the sour cream, the whole eggs, egg yolk, salt and vanilla. Gradually stir in 4 3/4 cups of the flour until a soft, sticky dough forms. Scrape the dough out onto a floured surface and use a pastry scraper to knead the dough until smooth, adding as much of the remaining flour as necessary. The dough should be soft and slightly sticky, but smooth and elastic.