No, you cannot. In order to board the plane, you must show the approval letter to the airlines. This letter is also compulsory in order for you to get the visa at the Vietnam airport. Therefore, we recommend that you leave plenty of time to apply for the visa to avoid having to delay or change your flight.
Can I travel without the letter and get the visa when I arrive?
Vietnam Visa India - About Visa on Arrival - Can I travel without the letter and get the visa when I arrive?