Unless: trừ khi, trừ phi

Trừ khi + a sentence means "unless": Mai em đi Vũng Tàu, trừ khi trời mưa, I'm going to Vũng Tàu tomorrow, unless it rains. It usually comes after the main sentence and adds the one case that would change things.

Trừ phi means the same but sounds bookish; you'll see it in older writing and now and then in careful speech. Trừ khi is the one to use.

Don't confuse it with trừ on its own, which goes before a noun and means "except": Ai cũng đi, trừ Nam, everyone's going except Nam. Before a whole sentence, one with its own verb, you need trừ khi.

You can often say the same with nếu … không: Mai em đi, nếu trời không mưa. Trừ khi puts the exception last, almost as an afterthought, which suits warnings and conditions: Đừng gọi giờ đó, trừ khi có chuyện gấp.

It can also open the sentence, with chứ (otherwise) bringing in the main part: Trừ khi anh đi cùng, chứ em không đi một mình, unless you come with me, I'm not going alone.