"Cap" is Hokkian languages which means "mix"
"Chai" is also Hokkian languages which means "vegetables"
So, basically, Cap chai is kind of food that consists many vegetables. Though "Cap Chai" is chinese (Hokkian) languages, but this food is one of Indonesia ethnic food. I don't know where is the exact place this "Cap Chai" created, but my opinion is North Sumatra, Indonesia. I said that because as far as I know, most Chinese people in Medan speak Hokkian.
My Dad, who was born in Riau, Sumatra, said that this Cap Chai (Indonesian call it Cap Jay) is definitely not from Riau. He said Medan-North Sumatra maybe the place where this food came from since so many Chinese-Hokkian food was found in Medan. D.I Aceh (above North Sumatra) is impossible too since almost the citizen is moslem and not speak Chinese. I found that not many Chinese live in Jambi. So, I skip Jambi. About West Sumatra... The Capital of West Sumatra is Padang, right? Padang is famous because of Padang Cuisine which is hot and spicy. Okay, maybe we can skip West Sumatra again. Why? Because Cap Chai is sweet and not spicy! About Palembang... I only know that most of Palembang ethnic food has sour taste and little bit spicy. Skip! Lampung? Hmm... I read that its ethnic food is similar with Palembang. How about Java? It maybe possible. I've heard about Cap Chai a la Semarang (Semarang is located in Central Java), but my grandma refuse that Cap Chai is from Semarang.
Note : Cap Chai is not made from vegetables only. People can add everything that they like, such as chicken, sausages, mushroom, tempura, etc. Hmm.. this food remembered me of pizza.. The truth is, pizza was made by add anything on bread, right? I think...."add-anything" concept in Pizza is similar with this Cap Chai