Travel itinerary...is it ok? wht procedure should i follow?

Mark, I have to inform you that the Vietnamese citizens are not yet eligible for 240 hour visa free transit scheme, so your itinerary is not eligible for this particular visa free transit scheme. In the case if you are a citizen of one of 54 countries that are eligible for 240 hour visa free transit, but it happens that you just live in Vietnam, then your itinerary could become eligible, but you should take care of one detail. You can depart from Hong Kong toward mainland China, but only by air or by a direct ferry to Shekou port (in Shenzhen) or to Nansha port (located south from Guangzhou and connected by metro to the city center). The only detail that you should have in mind before you take a ferry to one of those two ferry ports, is your confirmed flight ticket to Hanoi. You will be asked to present it by both Hong Kong and mainland Chinese immigration authorities. So, upon your arrival in Shekou/Nansha, you will apply for a temporary entry permit by presenting your flight ticket back to Hanoi and it is highly advisable to have at least the first night booked in advance because you will be asked for your address in China. This application is free of any charge. After the permit is granted, you will head to the city and enjoy your time in the way as you wish. Have a nice journey.