Combine the orange juice, carrot juice, sugar, and salt in a medium saucepan. Bring to a simmer and stir until the sugar dissolves. Remove from the heat and whisk in the lemon juice, orange liquor or vodka (if using), vanilla, and coconut milk. Transfer to a bowl, cover, and chill in the refrigerator until very cold, at least 4 hours and preferably overnight. Process in an ice cream maker according to the manufacturer's instructions. Transfer to an airtight container and freeze until firm. Sherbet is best eaten within one week.