Preheat oven to 170°F, or the lowest setting on your oven that is closest to this temperature. Move your oven rack to the top third of the oven. Line a cookie sheet with parchment paper or a silicone baking mat.
Combine raspberries and sugar in a small saucepan over medium-high heat. Stir often, crushing berries with the back of a wooden spoon as they cook. As soon as the rasberry mixture becomes syrupy, remove the pan from the heat. Strain the berry mixture through a fine-mesh sieve into a measuring cup, pressing down on the berries with a rubber spatula to extract all the juices. You should have about 1/2 cup of raspberry sauce. Discard the seeds and stir the vanilla extract into the sauce. Allow to cool for 10 minutes before proceeding.
Place egg whites in a mixing bowl and sprinkle the cream of tartar over them. Use a mixer to beat the egg whites until they hold stiff peaks.
Stir the cooled raspberry sauce, then gently fold 1/3 of the sauce into the beaten egg whites. Repeat two more times with the remaining 2/3 of the raspberry sauce.
Spoon meringue into a gallon-size zip-top baggie, forcing the mixture to one corner of the bag. Snip about 1/4" from the corner of the bag so meringues can be piped onto the prepared pan. Pipe out 1 1/2 inch swirled circles onto baking sheet, making sure that the meringues don't touch. If you prefer, meringues can be gently scooped and dropped by rounded teaspoonfuls instead of piping.
Bake for 2 hours at 170°F, rotating the baking sheet after 1 hour so they will dry evenly. When done, the meringues should not have any browning, and they should be dark pink in color.
Turn off the oven and allow meringues to remain in the oven for at least two hours to finish drying, or you can leave them in the oven overnight.
Meringues can be stored in an airtight container for 5 days, or they can be frozen for up to one month. If frozen, allow to thaw to room temp before serving.