In a small bowl, whisk the vinegar into the cornstarch. Whisk 2/3 cup of the yogurt into the vinegar mixture. Pour the yogurt and vinegar mixture into a small saucepan and cook over high heat, whisking constantly, until it comes to a boil. (The yogurt mixture will thicken very quickly.) When it is very thick, scrape it into the bowl of a food processor.
Blend the yogurt mixture for about 1 minute. Turn off the food processor and scrape down the sides of the bowl. Continue to blend the yogurt mixture until it is slightly cool and very smooth, about 3 more minutes. Add the remaining yogurt, mustard, sweetener, and salt to the mixture. Blend for another minute. Scrape down the sides of the bowl and blend for 30 more seconds.