I was in Vientiane a couple weeks ago, and my friends recommended this new vegetarian restaurant that just opened in October 2012.
So I decided to check it out, and I was delighted!
The staff was courteous, and helpful. Lucky for me, my friends speak Lao! But despite the language barrier, it was easy to order, just point at the buffet and you get what you want!
The portions were generous. I enjoy spicy food, so I enjoyed the Laos style dishes. You could ask for white or brown rice, which was great! They had a nice selection of cool drinks (bottled water, and boxed juices & soy milk / rice milk, from Thailand in a cooler).
They have a western-style toilet, in the back, through the kitchen! It's a smoke free environment, in a family friendly part of town.
I would recommend Manna Vegetarian Restaurant, and when I come back to Vientiane, I'll be back to try other Lao vegetarian dishes!
If they only had cooking classes . . .
Cons: Hard to find, Staff speaks Laos