I must revise my previous comment. I can find no visa on-arrival for a Vietnam entry available at the Moc Bai crossing either. Can someone provide a link which shows otherwise?

I don't know where you are, or why you have to have a visa on arrival, but there isn't one for border crossings. What the sign probably meant was there is an agent to assist you at the border. Like the General said, they used to have, and sometimes they do have, but now they do not have visa on arrival.

What is your problem with giving it to an agent and get your visa lickety split?

I guess the answer is: No visa upon arrival at borders crossings. But there are agents to get it done.