Warsaw is 5 hours behind Hanoi
Best overlap: 1:00 PM to 6:00 PM in Hanoi is 8:00 AM to 1:00 PM in Warsaw.
Both offices are open during 1:00 PM to 6:00 PM Hanoi time, which is 8:00 AM to 1:00 PM in Warsaw. Roughly 5 hours of common time means scheduling is manageable; just avoid the very edges of the window. The window above shifts seasonally: Warsaw keeps daylight saving while Hanoi ignores it.