Despite the Vietnamese calling it that, there is no "visa on arrival" in Vietnam, at least as usually the term VoA is understood: a visa you can apply for when landing in the country, like, say, in neighbouring Laos. A "real" VoA is something that you apply for (and hopefully get) without any preparation, at the border, airport, etc.
If someone needs a Vietnamese visa (and Chinese citizens need one), it has to be applied for ahead of travel.
There are two ways to apply for a visa: the paper kind (either apply directly to the Consulate or though a travel agency), and online. Now online is actually two different things. There's the eVisa, which you apply for online, before leaving. You receive the visa itself, print it out, and use that to board your plane and clear immigration. And there's a pre-arranged visa, unfortunately named "visa on arrival", which you still have to arrange BEFORE leaving. You get an acknowledgment letter, which you have to print, then use at the arrival airport to pick up the actual visa, and THEN clear immigration. Which is kind of pointless. The only advantage of the pre-arranged visa over the eVisa is that the eVisa is only single entry, where the pre-arranged visa can be multiple. But that can be arranged too if you apply at a consulate.
So what your girlfriend needs to do is get online and apply for an eVisa. Once she gets it, she can take the printout with her to the airport, and then use it at immigration.