Thank you for your question!
In your case, you will travel to Vietnam by air, the best to get your Vietnam visa is applying for Vietnam visa on arrival online thanks to its convenience and cheap cost. You then can pick up your visa at Ho Chi Minh airport upon your arrival with your visa approval letter.
If your stay is maximum 31 days, you can apply for 1 month single visa. It is good enough since exiting the country over one-day visa expiration date is acceptable.