Do I still need fast track if I fly business class into Ho Chi Minh City?

No. Business class does not guarantee a faster immigration line in Vietnam. Some airlines give a priority lane or lounge access, but that varies, and at many airports the immigration queue is shared.

Fast track is optional in any case. If you fly business, ask your airline whether priority is included at your arrival airport. If it is, you may not need to pay for a separate service.

You still need a valid e-visa or an exemption, and the digital arrival card, whatever class you fly. Those are what immigration checks, not the cabin.

If you plan to buy fast track anyway, check it covers the terminal you use. See the Vietnam travel guide for arrival tips, and if you still need a visa, you can apply for your Vietnam e-visa online.