Nah, it was the Ming Dynasty that ruined the contribution of seafaring, mathematical, astronomy and other achievements from Song, Jing and Yuan Dynasty. Ming is actually one of the most horrible Chinese dynasty.
If you read the book (on google book) I gave you few week ago, you can see the differences between Ming and Qing, the trade in Qing is much more better than Ming.
To be fair, back in those days, all of the big empires (such as Ottoman, Mughal...) in the world were perished by the Westerners. It was not easy for China to maintain most of its territory and not being colonized by vile colonialists~
Meh, Vietnam is just a mini version of China~
Back in those days, Chinese people were well known as honest businessmen.
Sigh, they say reputation cannot be restored that easily.