72 hour visa with 76 hour stay

I am scheduled to be arriving in Beijing on the afternoon of March 21st at 13:00 hours from the USA on United Airlines . I am scheduled to leave 17:00 hours from Beijing to Ho Chi Minh City Vietnam on March 24th with Air China. So about 76 hours total. Am I OK to travel without an entry permit in advance? I am confused by what I read online with conflicting explanations so wanted to confirm. Thanks for the help as I would love to save this huge cost for my family of 5.