In large mixer bowl combine 1 1/2 cups of the flour and the yeast.
In saucepan heat milk, sugar, butter or margarine, and salt just till warm (115° to 120°) and butter is almost melted, stirring constantly.
Add to flour mixture in mixer bowl; add egg, lemon peel, and vanilla.
Beat at low speed of electric mixer for 1/2 minute, scraping sides of bowl constantly.
Beat 3 minutes at high speed.
Stir in as much of the remaining flour as you can mix in using a spoon.
Turn out onto a lightly floured surface.
Knead in enough of the remaining flour to make a soft dough.
Continue kneading till smooth and elastic (8 to 10 minutes).
Shape dough into a ball.
Place in a lightly greased bowl, turning once to grease surface.
Cover and let rise in a warm place till double (1 1/2 to 2 hours).
Punch dough down; turn out onto lightly floured surface.
Divide dough in half.
Cover; let rest 10 minutes.
Pat each half evenly into a greased 8x1 1/2-inch round baking pan.
Sprinkle half of the Streusel Topping over each coffee cake.
Cover; let rise in warm place till double (30 to 45 minutes).
Bake at 375° about 20 minutes.