WordNet records no synonym and no antonym for vested interest. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(law) an interest in which there is a fixed right to present or future enjoyment and that can be conveyed to another
groups that seek to control a social system or activity from which they derive private benefit
Synonyms, antonyms and the broader and narrower relations for vested interest come from Princeton University’s WordNet. Full sources and licences.