In case you get the visa issued already, the period of time that you can stay in Vietnam depends on the kind of Vietnam visa you get. The expiry date shown on the visa is the date by which you should leave the Viet Nam. If you want to stay more in Vietnam, you must extend your visa, Cheap Vietnam Visa suggest that you should extend your visa at least one weeks prior to the expiry date.
In case you have not applied for Vietnam visa, the maximum length of validity currently legitimate is 3 months. Thus, you should apply for 3 month visa first. Then, you can apply for a new visa (if you by any chance leave the country), or get the visa extended if you still stay in the country. Please note that you can get a several times of extension for your visa, the regulated longest extension period is 3 months each.
Please contact us for further assistance