There are international flights on Vietnam Airlines from Ho Chi Minh City to Phnom Penh:

From		To		Transit	Flight (VN)
City	Local time	City	Local time	None	840
HCMC	14:00	Phnom Penh	14:40
HCMC	11:55	Phnom Penh	12:45	None	817

Flight times and schedules may change. Please search on specific dates for actual schedule.

See also:

Return flight

Flights from Phnom Penh to Ho Chi Minh City

Other flights from Ho Chi Minh City to Cambodia

Flights from Ho Chi Minh city to Siem Reap

International flights from Hanoi

Flights from Hanoi city to Phnom Penh.
Flights from Hanoi to Siem Reap