If it is made with tomato, is it vegetarian?
No. The standard nam prik ong contains minced pork. If you are vegetarian, do not choose by the tomato colour; ask about the meat and seasonings too.
Nam Phrik Ong · น้ำพริกอ่อง
Meet nam prik ong, the sauce eaten with vegetables at a northern meal. It is a red dipping sauce made from tomatoes, minced pork and chilli. Dab a little on vegetables or have it with sticky rice and you get the savoury depth of pork and the acidity of tomato together. You could order it with sai ua and lay a northern table.

The acidity of tomato and the savoury depth of pork are joined by chilli seasoning. A soft sauce on crunchy vegetables brings different textures together. The heat varies from maker to maker.
Even a tomato sauce has minced pork in it.
Nam prik is a dipping sauce eaten in Thailand with vegetables or rice. The north has many nam prik, 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 laid out on a small table.
No. The standard nam prik ong contains minced pork. If you are vegetarian, do not choose by the tomato colour; ask about the meat and seasonings too.
You have it a little at a time with vegetables and sticky rice. Order it with grilled food such as sai ua and you can share several flavours.
If you would like to read more about the food and the regions behind it, these sources are worth a look. Ingredients and cooking methods can vary from one cook to the next.
Sharing image credit: Takeaway · CC BY-SA 3.0