The six month multiple entry visa is applied for businessmen only and this kind of visa is allowed for "work permit" in Vietnam. Within 6 months you can travel in and out the country at any time you want.
Three Month Multiple Entry Visa means the visa is valid in three months and you can go in and out Vietnam as any times as you wish within three months. This is a kind of business visa for businessmen.
What is a "Visa on Arrival"? This is most likely the easiest way to obtain your visa without having to chase down embassies, consulates and the like prior to your trip, and is a valid alternative when applying for a tourist visa.
