Sprinkle the yeast over the water in a large bowl, and stir gently to moisten the yeast. Stir in salt, sugar, margarine and 4 eggs, and beat well. Gradually mix in the flour, about 150g at a time, up to about 1500g, until the dough becomes slightly tacky but not wet. Turn the dough out onto a floured surface, and knead until smooth and elastic, 8 to 10 minutes.
Grease baking trays, or line with parchment and set aside.
Cut the dough into 4 equal-sized portions (or 2 portions to make 2 large loaves). Cut each portion into thirds for 3-strand plaited loaves. Working on a floured surface, roll the small dough pieces into ropes about the thickness of your thumb and about 15cm long. Ropes should be fatter in the middle and thinner at the ends. Pinch 3 ropes together at the top and plait them. Starting with the strand to the right, move it to the left over the middle strand (that strand becomes the new middle strand.) Take the strand farthest to the left, and move it over the new middle strand. Continue plaiting, alternating sides each time, until the loaf is plaited, and pinch the ends together and fold them underneath for a neat look. Repeat for the remaining loaves.
Place the loaves onto the prepared baking trays, and let rise until double in size, 1 to 1 1/2 hours.
Preheat an oven to 180 C / Gas 4. Whisk 1 egg with vanilla extract in a small bowl, and brush the loaves with the egg wash. Sprinkle each loaf with about 1 tablespoon of sesame seeds.
Bake in the preheated oven until the tops are shiny and golden brown, about 30 minutes. Let cool before slicing.