Originally, by nature: vốn (là), vốn dĩ

Vốn says what something was from the start. It goes before là or the verb, and it has two close uses.

With là and a noun, it says what someone or something originally was, usually before a change: Anh Tuấn vốn là kỹ sư, mấy năm nay mới chuyển qua mở quán cà phê. Tuấn was an engineer to begin with; only in the last few years has he opened a café. Khu này vốn là ruộng lúa. This area used to be rice fields.

Before an adjective or a verb, it says what someone or something has always been like, often to explain something: Chị Lan vốn ít nói. Lan has always been quiet. Vốn đã means "already, to begin with", and the next part often adds something worse: Đường này vốn đã kẹt, giờ còn đào lên sửa nữa. Vốn dĩ is stronger, "in the first place, by its very nature": Chuyện này vốn dĩ không phải lỗi của em.

You'll meet vốn in news and stories, and in careful speech. In casual Saigon talk, hồi trước (before) often does the first job and trước giờ (always, until now) the second.

Compare vẫn (still): vẫn says nothing has changed since some point, while vốn goes back to how things were at the start. Từng (once) only says it happened at some point: Anh Tuấn từng làm kỹ sư, Tuấn once worked as an engineer. And don't confuse it with the noun vốn, capital or a stock of something: vốn đầu tư (investment capital), vốn từ vựng (vocabulary).