You can get a Vietnam Visa at the Vietnamese consulat in Luang Prabang. As there are some options to cross into Vietnam from Laos by bus, it is a good opportunity to obtain your Visa here.
The most important thing – it is dead easy. The ‘negative’ side – you have to stay a few days in LP (which is not a bad thing in my opinion).
Before heading to the embassy make sure you have the following with you: a valid Passport for 6 month , US$ and a passport picture. If you have none of the latter two, no worries. You can get US$ at the money exchange places (please be aware not to pay any fees – just exchange Kips to US$). If you don’t have a photo, please note that the consulate has no facility to take a picture (even though my guesthouse advised me they do). I only learned this is not the case when I arrived at the consulate. So it is better to get some pictures taken in one of the photo shops (there is one on the way to the consulate.