Do I need Vietnam airlines to notify immigration or do I apply for 72 hour visa on my own in Guangzhou?

I only have a single entry to China - flying from NY>Guangzhou staying for 1 week and then to Hanoi for one week(flying back to Guangzhou) Can I apply for 72 hour exemption in Guangzhou? I have a return ticket from Guangzhou to NY (but not leaving Guangzhou 24 hours after returning from Hanoi).