The Rimkhong riverside road has many restaurants selling cheaper food than the town centre. Nagarina and Smile Bar are two of the best among others. Many other food stall vendors are spread across the city selling excellent and cheap Thai and Issan food at Thasadej market and Sunday market. Nong Khai is famous for barbecued river fish and sticky rice.
There aren't many vegetarian restaurants here in specific, but some good choices are Yota Vegetarian restaurant serving many cheap and vegan curries and other chinese vegetarian food. Mut Mee and Ahan Jay are other spots for healthy and strictly vegan meals.