I have a son who have his own family in Vietnam. Now, how I take him and his family to the U.S. with me. I was the U.S. citizen already. Thank you!

You can apply for a green card for him as the son of an American citizen, but depends on his age and marital status, he need to get in line and wait for the priority date for years. Go to the Homeland Security Website for more info on immigration visa.