You must be in possession of a passport still valid for at least 6 months and possibly an entry visa to visit vietnam. Ensure you have to get a residence permit or not an entry visa or not. If you are a foreigner residing in France, check the date of validity of your residence permit not to be blocked once back in France. Generally, it is up to you to manage consular formalities. Delays in obtaining the visa vary from consulate to another, anticipate these efforts as soon as possible.
If you are a French citizen, you can make a request to a town hall or prefecture. If you are abroad, go with the Embassy of France in your country of residence.
Visa for Vietnam:
To obtain an entry visa, you must contact the embassy of the country of destination. Select a visa depending on your needs (length of stay and with one or more inputs). You can ask for 1 month visa if you stay there less than 1 month or 3 months visa if oyur stay are longer than 1 month. There are also 2 options for 1 and 3 months visa : one entry or multiple entry
If you go though an organization for Visa on arrival , contact to know better the procedures to follow. But it should be 2 steps - Require an invitation letter - You mus provide to your visa approval letter providor one copy of your passport - Get your visa on arrival ( in Hanoi Hochiminh city and Danang )
Many travel agencies propose this service but the cheapest one must be Vietnam visa service by Minh Anh Travel