Make the Jam and Syrup: Mash grapes with sugar and vitamin C powder, if using. Cover bowl and let stand at room temperature for 1 hour or refrigerated overnight.
Transfer grape mixture to a small saucepan and bring to a boil over medium-high heat. Reduce heat and simmer until thickened and grapes burst, 5 to 10 minutes. Strain liquid through a fine-mesh sieve into a bowl. Set sieve over another bowl and, using a plastic bowl scraper, press the grapes through the sieve to remove seeds and skin and create a jam. You should have about 1/2 cup liquid syrup and 3/4 cup grape jam. Cover and transfer both bowls to refrigerator for at least 1 hour and up to overnight.
Make the Custard: In a large bowl, whisk together egg yolks and 6 tablespoons sugar until pale yellow; set aside.
In a large saucepan, whisk together milk, cream, remaining 6 tablespoons sugar, vanilla bean, and seeds. Bring mixture to a boil over medium-high heat. As soon as mixture begins to rise up the sides of the pan, remove from heat.
Whisk 1 cup of the hot milk mixture slowly into the reserved egg yolk mixture; whisk to combine. Slowly whisk egg mixture into saucepan with remaining hot milk mixture; whisk in salt. Transfer custard mixture to a large bowl; cover and chill until very cold, at least 8 hours and up to overnight.
Add grape syrup to chilled custard and whisk to combine; strain through a fine-mesh sieve and discard solids.
Transfer grape jam to a large metal bowl and transfer to freezer; freeze until firm but not solid.
Transfer custard to an ice cream maker; freeze according to manufacturer's instructions. Transfer ice cream to bowl with semifrozen jam and fold with a rubber spatula to combine. Transfer to an airtight container and keep frozen until ready to serve, up to 5 days.
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