Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Vietnam to Brazil as you normally would.
Yes, there are multiple flights from Vietnam to Brazil for under 41,301,300 ₫. The cheapest flight booked recently is on American Airlines for just 41,301,272 ₫, but on average you can expect to pay 107,024,992 ₫.