[{"_id":"vencimentos","vencimentos":[[1899.99,241.92,869.99,696.11],[2301.45,100],[1468.12,85.9],[14.1],[268.59,27.6,428.51,173.85],[627.29,241.92,413.47,229.74],[1687.58,100],[241.11]]}]

db.collection.aggregate([{$addFields:{vencimentos:{"$reduce":{"input":"$vencimentos","initialValue":[],"in":{"$concatArrays":["$$this","$$value"]}}}}}])