viewpoint has 4 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 mental position from which things are viewed
“we should consider this problem from the viewpoint of the Russians”
a place from which something can be viewed
“from that vantage point he could survey the whole valley”
Synonyms, antonyms and the broader and narrower relations for viewpoint come from Princeton University’s WordNet. Full sources and licences.