Warm the sugar and water in a small saucepan, stirring until dissolved, 2–3 minutes. Let cool completely.
Blend raspberries, cooled syrup, lemon juice, and salt until silky. Add vodka or corn syrup if using.
Strain the puree through a fine-mesh sieve to remove seeds. Press to extract as much liquid as possible.
Chill the mixture in the refrigerator until very cold, at least 1 hour.
Churn in an ice cream maker until softly frozen. Transfer to a container and freeze until firm.
No machine? Pour into a shallow pan and freeze, stirring every 30 minutes until smooth and scoopable.