DRUGS - Transporting illegal drugs in or out of Vietnam carries a maximum penalty of death.
Don't be deceived.... in spite of the significant number of foreigners you will see smoking cannabis in the streets, it is not legal to do so. Many incorrectly believe it is.  The penalty if arrested by police can result in a very long time in jail.
PROSTITUTION and PORNOGRAPHY - Highly illegal.
PUBLIC DECENCY LAWS - There are no nude beaches. Topless women can and have been arrested, but not often. Even western women wearing sexy or revealing clothes in the conservative city of Hanoi will be called a slut and/or a prostitute and treated with contempt.
SPEAKING AGAINST THE GOVERNMENT - No freedom of speech in Vietnam. If you voice your opinion of the Vietnam War, Uncle Ho, corrupt police or government officials or the evils of communism, you risk arrest, time in jail and deportation.
CO-HABITATING WITH THE VIETNAMESE - The attitude to the law is relaxing over recent years, but the laws are still on the books. It is illegal for a foreigner to share a room with a Vietnamese of the opposite sex without a certificate of engagement or marriage.
ONLY VIETNAMESE CAN HAVE A VN DRIVERS LICENSE - By law, only Vietnamese people can obtain a Viet Nam drivers license. Some foreigners through a little bribery or kind favours have been able to get one. Your foreign drivers license has no validity in Vietnam. This includes the "International Drivers License". Vietnam does not recognise them at all.
PAYING THE POLICE - If you want a police report for your travel insurance to make a claim for that stolen camera or missing cell phone, expect to pay a service fee for them to do so. This is not a form of bribery or corruption, but it is their legitimate  service fee and will issue a receipt.
CRIMINAL DECEPTION, NOT A POLICE MATTER. - If you get ripped off by a taxi driver, overcharged or tricked into paying higher fees at your hotel, the Police are not interested. Those are regarded as civil matters of dispute and not a crime.

1.

Smoking or transporting drugs is prohibited in Vietnam

2.

Swearing at the government or having reactionary words

3.

Charity stealing will be blocked

4. 

Indiscriminate speech, gambling addiction is prohibited

5. 

Gambling is prohibited in Vietnam