If it is made with tomato, is it vegetarian?
No. The typical nam prik ong has minced pork in it. If you are vegetarian, do not choose by the tomato color alone. Ask about the meat and the seasonings together.
Nam Phrik Ong · น้ำพริกอ่อง
Meet nam prik ong, which is eaten with vegetables at a northern Thai meal. It is a red dipping sauce made with tomato, minced pork and chilies. Dab a little on vegetables or eat it with sticky rice, and you get the savory depth of pork and the sourness of tomato together. Order it with sai ua and set a northern Thai table.

Chili seasoning is added to the sourness of tomato and the savory depth of pork. When you pair the soft sauce with crunchy vegetables, the different textures come together. The heat varies from place to place.
Even this tomato sauce has minced pork in it.
Nam prik is a dipping sauce that goes with vegetables or rice in Thailand. There are several kinds of nam prik in the north, and nam prik ong is the kind that uses tomato and pork. You can also meet it with sticky rice and vegetables at a khantoke meal, where food is set out on a small table.
No. The typical nam prik ong has minced pork in it. If you are vegetarian, do not choose by the tomato color alone. Ask about the meat and the seasonings together.
It is eaten a little at a time with vegetables, sticky rice and similar foods. If you order it with grilled dishes such as sai ua, you can share several flavors.
If you want to read more about the food and the regions behind it, these sources are a good next step. Ingredients and cooking methods can differ from one place to the next.
Sharing image credit: Takeaway · CC BY-SA 3.0