Place frozen or fresh raspberries, 3/4 cup water, 1 cup sugar and salt in a medium saucepan. Over medium heat, bring mixture to simmer, stirring occasionally, about 7 minutes. Do not boil.
Press cooked raspberry mixture through a sieve or fine strainer, getting as much juice as possible. Discard raspberry seeds and pulp.
Mix in lemon juice and vodka. Cover raspberry puree with plastic wrap and refrigerate overnight, or place in the freezer until very cold, about 45 minutes. Don't allow it to freeze.
In a medium bowl whip cream until soft peaks form. Once raspberry mixture is cold (about 40 degrees), pour it in a steady stream into whipping cream, while whisking continuously.
Pour mixture into the canister of ice cream maker and follow manufacturer's instructions. Time will vary depending on the type of ice cream maker you have.
Once sherbet has finished churning, transfer to a container and freeze 3 hours to overnight.