Flying from China with the visa approval letter

Hi
I'm planning on flying to the Hanoi airport at the end of June. The only thing is that I am flying from Shanghai, China, and I don't know if having the approval letter is enough for getting through the China customs. I've read somewhere that a Chinese citizen with the approval letter must fly to Vietnam from a third country. Is that true or am I worrying too much? Thank you!

ps. I'm a Chinese passport holder