Using an electric mixer fitted with the whisk attachment, beat the egg whites until they turn foamy. Whisk in the salt, cream of tartar and vanilla on medium-high speed until soft peaks form, about 1 to 2 minutes. With the machine on, slowly add half of the sugar, about 1 tbsp. at a time (the mixture will start to thicken). Increase the speed to high and continue to whip, slowly adding the remaining sugar until the batter begins to look very stiff and glossy and forms firm peaks, about 5 minutes.
Preheat the oven to 225 degrees . Line 2 cookie sheets with parchment. Working quickly, drop 12 heaping tbsp. of the batter about 2 inches apart onto each pan. Lightly press a sour cherry into each meringue, then cover the cherry with about 1 tsp. more meringue. Using your fingers, seal the 2 meringue dollops together. Bake for 2 hours, then turn off the oven and let the meringues sit in the oven (dont open the door) for an additional 20 minutes. Serve the cookies within 24 hours.