Hi. When we talk with a group of people(plural),we say:"Et vous?" But when we talk with one person it depends of the relationship between the speakers. If you consider you partner as a friend or a member of you family,you say:"Et toi?".It's the informal way.But when you speak with your boss for example or a person wich is older than you,you must keep an amount of respect.You must say:"Et vous?".It's the same thing with strangers:people that you don't know. I hope that I've been useful to you.Tell me if it's okay!

Bonjour "Et toi ?" It is used when you know well the people you are talking to ... It is informal way "Et vous ?" is used when you don't know the people you are talking to... It is the formal way or polite way and could be also the plurial.

the diference is the degree of politeness. "et tu??" is the informal way used to talk friends, and family for example, "et vous??" is more formal form, it's more polite. But there is not diference in the meaning in that case. "Et vous?" also is used to a group of people as you(plural), here the meaning changes.

It depends who you are speaking to. EDIT: somebody gave me a negative despite the fact that my comment is 100% correct.

formel - informel