Foreigners when enter Vietnam with commercial or trading purposes, when the deadline of their visa is near, you can ask for an extension of a one month visa to carry out their commercial activities in Vietnam.
Under this circumstance, a Vietnamese company or organization has to act as guarantee for that foreign individual at the Vietnamese Ministry of Security and Immigration office at the city that individual is staying at.
Depends on the type of visas the foreigners applied for before that the Immigration office will decide on giving you an extending visa stamp or a new visa stamp.