tại sao = vì sao = why
tại vì = because
tại is more about where the root problem is.
vì is more about the reason and who causes it.
So it's like "why is your dad yelling at you?"
Tại I have bad grades = more about facts
Vì I didn't study = more about reasoning
Most people use them interchangeably though. I just want you to have a clear understanding of the words.