Learn Russian: Как вас зовут?

Because Russians don't ask 'What is your name?' The literal translation of this sentence is 'How do they call you?' Therefore, it starts with Как, meaning 'how'.

Why is it вас instead of вы?

вас is the object form (Accusative case) of вы (you). In the literal phrase 'How do they call you?', 'you' is the object receiving the action of being called, so it changes form just like 'he' changes to 'him' in English.

Where is the word for 'they' in this sentence?

There isn't one! The verb зовут means 'they call', but Russian often leaves out the word for 'they' when making a general statement about what people do. It is similar to saying 'How are you called by people?' in English.

Is this a formal or informal way to ask for someone's name?

This is the formal or plural way, because it uses вас. If you are talking to a close friend, a child, or someone your own age in a casual setting, you would change вас to the informal тебя, making the question Как тебя зовут?