From Sydney to Ho Chi Minh City, you can travel with Singapore Airlines, Thai Airways, China Southern Airlines, or Vietnam Airlines. 

Vietnam Airlines (direct flights, daily flights except for Monday):

Depart Sydney: 10:15 AM
Arrive Ho Chi Minh City: 4:00 PM
Total travel time: 8 hour 45 minutes

Vietnam Airlines Baggage Allowance
Vietnam Airlines Meals
Vietnam Airlines Contact

Singapore Airlines:

Depart Sydney: 9:10 AM
Transit in Singapore: 2:10 PM - 5:35 PM 
Arrive Ho Chi Minh City: 6:40 PM 
Total travel time: 13 hours 30 minutes

Singapore Airlines
Singapore Airlines Baggage Allowance
Singapore Airlines Meal 

Singapore AirlinesContact

CATHAY PACIFIC:

Depart Sydney: 08:35 AM
Transit in Hong Kong: 3:10 PM - 4:20 PM
Arrive Ho Chi Minh City: 5:50 PM
Total travel time: 13 hours 15 minutes

Cathay Pacific Airways
Cathay Pacific Baggage Allowance
Cathay Pacific Meals
Cathay Pacific Contact

CHINA SOUTHERN AIRLINES (usually has the cheapest flights on this route):

Depart Sydney: 10:50 AM
Transit in Guangzhou: 5:00 PM -> 9:30  PM
Arrive Ho Chi Minh City: 10:45 PM
Total travel time: 15 hours 30 minutes