Place the egg yolks in a medium, heat-safe bowl. Place them to the side of the stove, within reach.
Place a separate large bowl on the side with a fine mesh sieve positioned above it.
In a medium-size, heavy bottom saucepan, place the granulated sugar, rice flour, salt and just enough milk to create a thin paste (about 1/4 cup), whisking constantly until smooth. Add the rest of the milk and whisk again until smooth.
Place the saucepan over medium-low heat and, whisking frequently, heat until just beginning to simmer. Drizzle about 1/2 cup of the hot milk mixture into the bowl with the egg yolks
Scrape the egg yolks with the added milk mixture into the saucepan. Cook again over medium-low heat, whisking frequently, just until the mixture thickens enough that the whisk leaves a visible trail in the custard (about 3 minutes).
Working quickly, scrape the custard into the sieve above the bowl. Stir to pass it through the sieve, then whisk in the vanilla extract until smooth. Don’t try to pass any solid bits through the sieve.
Place plastic wrap directly on the surface of the hot custard and allow to cool to room temperature.
Refrigerate with the plastic wrap on top for at least 2 hours and up to 3 days.
Prepare and assemble the trifle
Slice the prepared pound cake into 3/4-inch square cubes, and set it aside.
Wash all of the berries and pat them dry. Set the blueberries aside.
Hull the strawberries by slicing off the greens and any white tops. Slice enough of the strawberries vertically as necessary to get 20 vertical slices to display on the edge of the trifle bowl.
Chop the remaining strawberries and place them in a separate, medium-size bowl. Add the white wine vinegar, toss to coat, and set aside.
Line the bottom of your trifle bowl with the squares of cake in a tightly packed single layer of cubes.
Remove the custard from the refrigerator, and uncover it. Whisk the custard vigorously until smooth, and spoon or ladle half of the cream on top of the cake cubes. Spread the custard into an even layer.
Stand up the strawberry slices against the side of the bowl along the perimeter, nestled into the custard. Top with half of the chopped strawberries and half of the blueberries.
Top with another tightly packed single layer of cake cubes.
Top the cake with another layer of custard, and repeat the sliced strawberry procedure.
Top with the rest of the chopped strawberries and the remaining blueberries, but reserve a few blueberries for decoration.
Cover the trifle tightly, and refrigerate the trifle for at least 2 hours and up to overnight.
Right before serving, place the cold whipping cream in the bowl of a stand mixer fitted with the wire whisk or a large bowl with a hand mixer and whip until soft peaks form.
Add the confectioners' sugar, and whip again until stiff, but not dry, peaks form.
Uncover the trifle, top with the whipped cream in an even layer, and top with the reserved blueberries. Serve chilled.