Is it vegetarian if it is made with tomatoes?
No. The typical nam prik ong contains ground pork. If you are vegetarian, do not choose it by the tomato color alone. Ask about the meat and the seasonings too.
Nam Phrik Ong · น้ำพริกอ่อง
Meet nam prik ong, a sauce eaten with vegetables at Northern Thai meals. It is a red dipping sauce made from tomatoes, ground pork and chilies. Dab a little on vegetables or eat it with sticky rice, and you taste the savory depth of pork and the tang of tomato together. Order it with sai ua to set a Northern Thai table.

Chili seasoning is added to the tangy tomato and the savory pork. The smooth sauce goes with crunchy vegetables and the two textures work together. The heat differs from place to place.
Even a tomato sauce has ground pork in it.
Nam prik is a dipping sauce eaten with vegetables or rice in Thailand. The North has many kinds of nam prik, and nam prik ong is the type made with tomatoes and pork. You can find it with sticky rice and vegetables at a khantoke meal, where food is set out on small tables.
No. The typical nam prik ong contains ground pork. If you are vegetarian, do not choose it by the tomato color alone. Ask about the meat and the seasonings too.
You eat it a little at a time with vegetables, sticky rice and so on. Order it with grilled dishes such as sai ua and you can share a range of flavors.
If you want to read more about the food and the regions behind it, these sources are a good place to start. Ingredients and recipes can vary from one restaurant to the next.
Sharing image credit: Takeaway · CC BY-SA 3.0