[{"_id":"...","vector":[100,50,4000]}]

db.collection.update({_id:"..."},[{"$set":{"vector":{"$let":{"vars":{"delta":[-50,0,-2000]},"in":{"$map":{"input":{"$zip":{"inputs":["$vector","$$delta"]}},"as":"tuple","in":{"$sum":"$$tuple"}}}}}}}])