To remind the Vietnamese that anti-Chinese protests against Chinese sovereignty in the South China Sea carries a price.
China has imposed the same penalty on the Philippines.
These two idiot countries will understand that vocally opposing China carries an economic and possibly military price. You do not screw around with China lightly.
If I remember correctly the case with philippine banana was at least found to be substandard after inspection may it be an excuse and then was rejected volunteerly by the buyers in China more than it was a direct order by the government. The Vietnamese case is different as in the article it says nothing about the quality of those vietnamese goods.