On the plane.
Ho chi minh airport is quite small. You can see this counter after you exit the area which you pick up your luggage.
There are various package available for both sim, micro simcard and nano simcard.
You can pay in both USD or VND.
The person at the counter will help you set up your phone.
If you just need the data plan, is only 3 USD.
We went out and enquired about the taxi rate. It is only around 1 USD difference if we book from the taxi counter inside the airport. Therefore, we decided to take the taxi from the taxi counter as it is much safer.