Good moral character generally means that you don’t break the law and you meet all of your legal obligations, such as paying taxes and making child support payments. If you have a criminal conviction, consult with an immigration attorney. Members of the military must show that they can read, write, and speak English. They also must show knowledge of U.S. government and history, called civics. You’ll have to pass exams in both English and civics. You can find information about the exams online. If you served in peacetime, you can apply for naturalization if you meet the following requirements:  Serve honorably for at least one year. Obtain a green card. File an application while still in service or within six months of separation. The requirements are different if you are serving during a period of hostilities. The U.S. has currently been in such a period since 2002, and it will last until the President designates that it is over. In this situation, all members of the military can immediately apply to become naturalized. Every installation should have a person to contact. They will usually be in personnel or the Judge Advocate General’s Office. You will need to complete Form N-400 and Form N-426. Contact them and receive a packet of information. Your filing fee is waived.  USCIS has customer service specialists who can answer questions from military members and their families. Call 1-877-247-4645 Monday through Friday, 8:00 am to 4:00 pm. You can also email militaryinfo.nsc@dhs.gov. Before you can become a citizen, you must show attachment to the U.S. Constitution by swearing an oath of allegiance.

TL;DR in Vietnamese:
Khỏe mạnh và nhân cách đạo đức tốt. Chứng mình trình độ tiếng Anh và giáo dục công dân Hoa Kỳ. Phục vụ trong thời bình. Phục vụ trong thời chiến. Xin quốc tịch. Tuyên thệ.