Is it a vegetarian dish if it is made with tomato?
No. The standard nam prik ong contains minced pork. If you are vegetarian, do not choose it by the tomato colour alone, but ask about the meat and the seasonings.
Nam Phrik Ong · น้ำพริกอ่อง
Meet nam prik ong, eaten with vegetables at a northern meal. It is a red dipping sauce made from tomato, minced pork and chilli. Dab a little on vegetables or eat it with sticky rice and you get the savoury depth of pork and the tang of tomato together. Order it with sai ua and set a northern Thai table.

The tang of tomato and the savoury depth of pork are joined by chilli seasoning. When the soft sauce goes with crunchy vegetables, different textures blend. The heat differs from maker to maker.
Even a tomato sauce has minced pork in it.
Nam prik is a dipping sauce eaten with vegetables or rice in Thailand. The north has several nam prik, and nam prik ong is the kind that uses tomato and pork. It can also be met together with sticky rice and vegetables at a khantoke meal, with food laid out on a small tray table.
No. The standard nam prik ong contains minced pork. If you are vegetarian, do not choose it by the tomato colour alone, but ask about the meat and the seasonings.
It is eaten a little at a time with vegetables, sticky rice and so on. Ordered with a grilled dish such as sai ua, it is easy to share several flavours.
If you want to read more about these dishes and regions, these sources are a good place to start. Ingredients and cooking methods can differ from one eatery to another.
Sharing image credit: Takeaway · CC BY-SA 3.0