That's why, therefore: vì vậy / vì thế

Vì vậy means "that's why" or "therefore". It starts the result, after you've given the reason: Tuần này em bận lắm, vì vậy em không đi được. I'm really busy this week, so I can't go. Vì thế means the same; Saigon leans to vì vậy, Hà Nội to vì thế.

It's vì (because) plus vậy (that): "because of that". So it always comes after the reason, at the start of the result, often after a full stop or a comma: Giá xăng tăng. Vì vậy, giá đồ ăn cũng tăng. Petrol went up. That's why food prices went up too.

You know nên (so), which joins the two halves into one quick sentence: Em bận nên em không đi được. That's the most common choice in conversation. Vì vậy puts a small pause between reason and result, so it suits explaining, at work and in writing. In notices, reports and the news you'll also see do đó. In relaxed Saigon speech, vậy nên and thành ra do the same job.

Vậy là is different: it tells what happened next in a story. Vì vậy gives the reason.