You only use "喂" when you are on the phone. Do not use it when you meet someone in person. Use ”你好" when you just meet someone. But, sometimes, you can use both of them on the phone, like "喂，你好!......".

你好 is the first greeting when you meet someone at any time , 喂 is used over the phone to make sure someone answer the phone.

喂 is only used to start a phone conversation.

yes ,it is what you think。

I think 你好 is like hello while 喂 is like hey. and 喂 is not so polite， I suppose