Preheat the oven to 425 F.
In a large bowl, mix the flour, sugar, baking powder, and salt.
Beat heavy cream and egg together in another bowl; pour into the flour mixture and stir with a wooden spoon until a soft dough forms. (If the mixture seems to dry, add more cream, 1 tablespoon at a time.)
Dump the dough onto a lightly floured counter. Pat into a round shape about 1 inch thick and press it out into a circle about 8 inches in diameter. Using a 3-inch biscuit cutter, cut out circles, rerolling scraps as necessary to get more circles. Place them on an ungreased baking sheet several inches apart.
In a small bowl, whisk one egg until blended. Brush the tops of the dough circles with the egg. Sprinkle evenly with the 1 tablespoon of sugar.
Bake for 35-40 minutes, or until light golden brown. Let cool on a pan or a wire rack until cool enough to handle, then transfer to a rack to cool until warm.
Meanwhile, stem the strawberries, and cut each berry lengthwise into four or five slices. In a medium bowl, toss the strawberries with the vinegar, lemon zest, and 6 tablespoons of sugar and let macerate for about 30 minutes.
Using a handheld mixer or whisk, whip the cream with the 1 tablespoon sugar and the vanilla just until it holds soft peaks.
Split each shortcake in half horizontally. Set the bottom halves, cut sides up, on individual serving plates. Divide the strawberries and their syrup evenly among the bottom halves, then top the strawberries with the whipped cream, again dividing evenly. Balance the top halves on the whipped cream and serve immediately.