Could only work if same airline and you staying in the transfer area. The plane (even if punctual), landing, passengers getting off, and you, even if allowed to, running to the departure gate from Arrival (through transfer area) in 20 min. (as the gate will probably close 20 min. before departure)... It just won't work! Book another flight and meet in Myanmar later!
My last two trips to Vietnam, the Don Muang immigration queues were quite short, and it didn't take longer than 10 minutes. Two hours before the flight is plenty, as long as you don't have a lot of luggage. Fortunately, Vietnam isn't as popular with Chinese tourists yet as Thailand is.