There are usually at least two carts where you can get some vegan or veg options. One lady makes the papaya salad and has Vietnamaese style veg spring rolls-usually. If you get there early she can make them without putting the scrambled egg in. Tell her no shrimp paste and you want it 'jay'=vegetarian. 10,000 kip papaya salad. 5,000 kip 4 spring rolls.
There is also another lady who has various pots and fish. But, she always has at least a couple of vegan dishes either the cooked green papaya, bamboo shoots and pickled veggies and she always has rice. Prices range from 3-5000 kip depending on portion size.
Also, for vegetarians there is often a woman who makes donuts, banana cake, taro cake, and brownies who studied in Vient. Her english is very good and her stuff looks tasty. Donuts/cake 3,000 kip
There are usually plastic tables and chairs out to sit and eat or take the food away.