Preheat the oven to 250ºF and line a rimmed baking sheet with parchment paper. Set aside.
Combine the egg whites, vitafiber, and salt in a large glass or metal bowl (if you have a stand mixer, use the bowl for it). Using a clean spatula (any oil will inhibit proper whipping) to stir to combine. Clumps will form and that is ok. Place the bowl over a pan filled with at least one inch of simmering water, ensuring that the water does not touch the bowl. Stir often to keep the egg from cooking onto the bottom of the bowl until the mixture reaches 165ºF. Stir constantly and maintain the temperature between 165-175ºF for 5 minutes and up to 8. The sweetener should be fully dissolved at this point and the mixture is light and fluffy.
Remove the bowl from the heat and using a handheld mixer (or transfer to the stand mixer), beat on high speed with a whisk attachment until the mixture forms stiff peaks that are white, shiny, and smooth, about 2-4 minutes. Add the vanilla and sprinkle the xanthan gum over the meringue and continue to beat until fully combined, another few seconds. The mixture will become a bit stiff.
Scoop the meringue out of the bowl and into the middle of the parchment-lined sheet. Using a spatula, spread into a 10-inch circle, making sure that the edges are a bit higher than the center (this will make it so you can fill it later one).
Bake until the meringue is dry to the touch and it lifts clean from the parchment, about 1 hour to 1 hour 30 minutes (try not to open the oven as much as possible to prevent cracks). Turn the oven off and leave the meringue inside. Prop the door open with the handle of a wooden spoon. Leave the meringue to cool completely for another 2 hours.
While the meringue cools, prepare the fillings. Stir together the strawberries, sweetener, vanilla, salt, lemon zest, and lemon juice. Let sit for 20 minutes.
Whip the cream, sweetener, vanilla, and salt to stiff peaks.
Carefully peel the meringue away from the paper and place onto a serving plate. Fill with the cream and top with the berry mixture. You can store the unfilled pavlova by wrapping it in plastic and keeping it in an airtight container for up to 3 days. Filled, the pavlova can stand for about an hour before slicing and serving.