I was last there two years ago, but at the time, you had to have everything fully organised ahead of time if going by a land border (i.e. visa sticker on the page), assuming you needed one.

If arriving at an airport (esp a major airport) then visa on arrival was possible, but you still had to organise everything online ahead of time.

I can't see anywhere that regulations have changed.

This link has a list of consulates, including in Kunming and Nanning. Since you'll almost certainly be going near/through one of them if travelling overland, you could get the visa there.

http://www.learn4good.com/travel/vietnamese_embassy_consulate_visa_guide.htm

Sorry I can't be more helpful.

From wikipedia travel guide to Vietnam:

Visitors from the following countries do not require a visa and can stay for the following number of days.

• 15 days: Denmark, Finland, Japan, Norway, South Korea, Sweden, Russia
• 21 days: Philippines
• 30 days: Indonesia, Laos, Malaysia, Singapore, Thailand

All other nationalities will require a visa in advance to visit Vietnam.

When I went you could get one there and that was two years ago. At the Airport there's a little desk in front of the customs line that you go to. Be prepared for a wait, it's honestly better to just get it here. 

I'm not sure, better to go prepared. I got one in two hours from Beijing, just better to try at the embassy first

Yes you can!  I did this last year.  If you want the safe bet, stop at the Vietnam Consulate in Kunming, they can process same day for 500rmb or next day for like 300rmb.  (I dont remember exact price.)  If you find yourself at the border town of HeKou without a visa to get into Vietnam, you can still handle it in HeKou.  Go to the bus station and start asking around.  There is a guy who lives there and usually hangs out at the bus station, his name is HeKou Li. He speaks english.  He will get you a legit visa same day for 500 rmb.  It gets processed on the Vietnam side of the border.  This might scary for you because you have to give him your passport and wont see it for a couple hours, but it worked for me. I talked with local police and what not before hand, so I was reassured.  He can also change money for you, get you a train ticket, or bus ticket. He ripped me off on the bus ticket, you wait till you are in vietnam to buy, people can speak some english.  On the other side of the border, you can ride a motorbike to the train/bus station, for like 20,000 vietnam dong.  (1 us dollar).  There is some cool traveling spots near here on the Vietnam side, you will run into backpackers and travelers at the train station if you have any questions.
 You will also get propositioned for prostitutes in HeKou, I recommend staying away, but whatever.  There is not much to do in HeKou while you wait, i went to a web bar.