Visa on Arrival

I'm reading off the official Vietnamese Embassy site and it says that Visa on Arrival is only good for certain situations,

Attending funerals of family members;
Taking care of family members who are seriously ill;
Providing technical emergency support; medical care for seriously-ill patients; natural disaster and epidemic relief in Vietnam;
Being invited by Vietnamese government;
Other emergencies."

Also, I do not see anywhere on the official site where I can apply for a visa on arrival. Can you explain ? Thank you very much for your time.