Bucharest is 5 hours behind Ho Chi Minh City
Best overlap: 1:00 PM to 6:00 PM in Ho Chi Minh City is 8:00 AM to 1:00 PM in Bucharest.
The overlap falls at 1:00 PM to 6:00 PM in Ho Chi Minh City, or 8:00 AM to 1:00 PM if you're in Bucharest. You've got a decent 5-hour window. Set a recurring time near 1:00 PM in Ho Chi Minh City (8:00 AM in Bucharest) and stick to it. Just remember Bucharest changes its clocks for daylight saving and Ho Chi Minh City doesn't, so confirm the overlap whenever the seasons turn.