The reason … is that: sở dĩ … là vì

Sở dĩ … là vì means "the reason … is that …". It explains something that has happened, or that everyone can see. That fact comes first, after sở dĩ, and the reason follows là vì: Sở dĩ quán này đông là vì giá rẻ. The reason this place is so busy is that it's cheap.

It sounds deliberate: it's how you explain yourself to a boss, in a presentation or in writing. Sở dĩ tôi chọn công ty này là vì gần nhà. The reason I chose this company is that it's close to home. In the news, là do often takes the place of là vì.

In conversation, people usually drop sở dĩ and keep là vì: Em tới trễ là vì xe hư. I'm late because my motorbike broke down (in Hà Nội, đến muộn, xe hỏng). The là gives the reason some weight, like "it's because …".

You know vì, which simply joins a result and its reason, and vì … nên, where the reason comes first. Sở dĩ is different: it goes before the result, never before the reason, and it never pairs with nên.