There is a small vegan food stand right in the middle of the Luang Prabang's Night Market alley. It is located on the left side of Wat Mai, when facing it from the Night Market. The sign says "Vegetarian buffet" but everything is actually vegan.
For 10,000 kip (around USD1.2), you can fill one plate with as much as you want. All the food is cold but you can ask the owner to heat your food up in the wok (no charge). The owner speaks basic English and is very nice.
You can also buy drinks (such as Beerlao, water and sodas) for an extra.
Overall, the dishes are the same every night but there can be small variations from time to time.
There is a long table next to the stand where you can sit to eat and socialize with other people. Some soy sauce and napkins are also on the table for everyone to use.
The stand opens at around 5:30pm, along with the rest of the night market.