In Vientiane, like in Thailand you can buy food everywhere.
The most striking difference is that many of the street shops sell barquettes with different kind of fillings, that is another sign of the French influence.
In Laos as in Thailand the people also eat insects but in Laos they have extended the assortment to include scorpions and poisonous spiders. I also saw a lot of food i could not identify.
How the taste is ? i do not know, i did not taste, i only took pictures. I normally never eat food from the streets, also not in Thailand. I know that much of the food is safe to eat, otherwise there would not be sold food in the street. There would be no people left if everybody died from eating it.
But i will not take the risk of getting food poisoning. Also you have to think of that if you go to another country you are not used to the bacterias there can be in the food. They can be safe for the locals to eat because they are used to them, but you are not and might get sick from them.
Even the vendor do his best to keep the food clean, you are still in the middle of the street, and there are no running water where the vendor can wash his hands or dirty dishes, and there is no refrigerator to store the ingredients.
It’s the perfect recipe for an upset stomach.
So if you want to fell the local beat and sit down in the street and eat with the locals, my advice is:
First make sure you have enough Imodium, the best medicine for diarrhea, and you are close to a useable toilet.
Imagine to walk around a 3. world city, far from your hotel, and suddenly get diarrhea and need a toilet right away, even if some nice shop owners let you use their toilet, you have to know a local toilet can be scary experience.