Pick a date and flight for each leg. Later legs only offer dates that keep at least 4 days in China and 4 in Vietnam, and the total updates as you go.
Every flight checked for each leg, grouped by date. The cheapest flight on each date is highlighted. Prices per person, economy.
You enter at Guangzhou and leave from Guangzhou or Shenzhen, so one entry is enough.
Apply online before the trip.