Place plums in a glass carboy used for brewing. Pour in 2 cups plus 2 tablespoons rectified alcohol and 2 cups plus 2 tablespoons vodka. Seal and store in a cool, dark place for 5 weeks.
Stir sugar into the carboy. Seal and store in a cool, dark place for 4 weeks.
Strain liqueur through a sieve lined with muslin cloth. Pour liqueur into bottles, seal, and store in a cool, dark place for at least 3 months before using.