I've obtained a number of Vietnam visas from their embassy in BKK for varying ports-of-entry. IME, they've all been overnight. I did take a look at the so-called visa-on-arrival (VOA) based on obtaining a pre-approval letter from several of the sources you noted, along with Vietnam Stay, and they pretty much all say the same thing: fill out a form with typical visa info, and they will then obtain and fax/send a letter for you to present on arrival. I can't vouch for this particular approach in Vietnam, but will say that I've been recently doing more VAOs in Asia recently than before... countries like Laos and Nepal, with success (extensively in China until they shut it down a few years ago due to reciprocity with the US).

If you decide to get VOA, please let us know how it worked out for you.

Happy travels