Combine all ingredients except the pears in a large skillet over medium-low heat, stirring until the sugar dissolves. Peal the pears, cut in half, and core. Place the pears in the poaching liquid, core-side down. Cover and let simmer gently for 15 minutes. Flip pears over, cover the skillet and let poach for another 5 minutes. Remove the pears, cut into slices, and place in a bowl. Strain the poaching liquid into the bowl, cover with plastic wrap, and refrigerate while preparing the pavlova.

Preheat the oven to 300 degrees. Using a bowl or pie pan as a guide, trace a circle onto a piece of parchment paper about 8 inches in diameter. Place the parchment, marked side down, on a baking sheet.

Place the egg whites, salt, and cream of tartar in the bowl of an electric mixer fitter with the whisk attachment. Beat on low speed until frothy. Raise the speed to high and add the sugar gradually, about 2 tablespoons at a time. Continue to beat on high until stiff glossy peaks form, about 8 minutes. Beat in the vinegar and vanilla.

Spread the meringue on the prepared baking sheet and, using the back of a spoon, form a well in the center. Bake for 1 hour and 15 minutes or until crisp around the edges and the center is fully set. Remove from oven and let cool. When the meringue is cool enough to handle, gently peel off the parchment paper and let cool completely on a wire rack.

Before serving, combine the cream and sugar in the bowl of an electric mixer and beat until stiff peaks form. Do not over beat.

Remove the pears from the liquid with a slotted spoon and pat dry. Fill the well of the pavlova with cream topping, place the pears on top, and sprinkle with the chopped hazelnuts.