WordNet records no synonym and no antonym for vicissitude. 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.
a variation in circumstances or fortune at different times in your life or in the development of something
“the project was subject to the usual vicissitudes of exploratory research”
mutability in life or nature (especially successive alternation from one condition to another)
Synonyms, antonyms and the broader and narrower relations for vicissitude come from Princeton University’s WordNet. Full sources and licences.