I have a green card 10 years and I live In USA, but I have a passport from Vietnam.

Ngan, you can apply for a tourist L type for China to visit your friend. You should first prepare your passport and copy of its data page, a completed application form, a passport-style photo, green card in USA and an invitation letter from your friend.
Next, go to the embassy of China to submit the application.

Is hard to get the visa? I just go about 4 days and i will fly to vietnam. What is paper to invitation? Passport china can i go for hongkong? If i want apply go hongkong. How much for the fee?

It is not that difficult to get that as long as you make full preparation.
You can find the sample of invitation from this website.
You cannot visit HK on a Chinese visa. You need to apply for a different visa for HK.