I ate at the restaurant floating on the river. (There's also one on the riverbank, in a garden, run by the same people). The setting was lovely (floating on the Mekong) and there was a pleasant ambiance with friendly staff. It was nice to see both locals and foreigners there, although it seems to be a place aimed mostly at tourists. The vegetarian section on the menu wasn't huge, but it had a decent number of dishes - traditional Thai curries and stir-fries. The garden restaurant menu is similar but a bit cheaper I think. I had a cashew nut stir fry and fried rice. They brought out fish sauce with the fried rice (in a dish on the side), even though I was very clear about being vegetarian and ordered off the vegetarian section. Well, that isn't an unusual occurrence in Thailand. Together with a fresh coconut, it cost me 215 bat, so not as cheap as some local places, but not unreasonable. Also, in their favour, this was the only restaurant I could find that had veg*n options that was open at night in Nong Kai.
Cons: Better understanding of vegetarianism needed, More variety of vegetarian dishes needed