Not sure which Passport holders can get a Visa on Arrival, but here is the process if you can, I have just applied for a Business Visa ( UK Passport) so assume a Tourist Visa will follow the same process?
Visa on arrival needs to be accompanied by a confirmation letter from the Immigration department, stating your VOA is approved. This can generally be arranged by a Tourist body / Travel Agent, in country, who will lodge the application on your behalf your Passport details will need to be sent to them in advance. They will then send a letter confirming the VOA is approved and validity etc.
Two photocopies of the approval letter should be hand carried with you, one copy of which needs to be passed to the Airline on check in, the second when arriving in Vietnam in order to collect your visa, plus a fee of $25 US.
As a precaution you should also have 2 x Passport photos, less than six months old as these may be required to support the two forms referenced above.
Visas are generally granted for one month (30 days)
Hope this helps?