Lao food is built around sticky rice, grilled meats, larb, green papaya salad and herb-heavy soups, with a French legacy of baguettes and coffee. Most savoury dishes use fish sauce or fermented fish paste, which is a challenge for vegetarians. If you eat vegetarian, say so clearly and ask what the stock and sauces contain, because a dish with no visible meat may still be seasoned with fish sauce.
Vientiane does have a small Buddhist-style vegetarian scene. Guides and directories list cafes and buffets such as Vegetarian in the Golden Age, Reunion Cafe and Khouadin Vegetarian, usually serving tofu, mushrooms and mock-meat dishes. The sources we opened describe most dishes as vegan but warn that some items may contain egg or dairy, and listings can be out of date, so we have not given any of them a Pure Veg or Vegan badge. Phone or ask in person before relying on them, especially on Jain, onion-free or strictly vegan diets.
Indian restaurants in the city, such as Nazim and Taj Mahal, appear in vegetarian directories as serving broad vegetarian menus, though they also cook meat and are usually halal-oriented. We could not open a page confirming any Pure Veg or Jain kitchen in Vientiane, so none is badged. Practical tips: carry some packaged snacks, ask your hotel to arrange simple rice, vegetable and egg or tofu dishes if you eat egg, and book Indian meals through your tour operator if you need strict arrangements.