Which visa do I need?

I'm going to Vietnam at the beginning of February for 3-5 months to travel around and visit friends. We've been thinking about traveling to other SE Asian countries but haven't decided yet. I've heard all different information about which visa to apply for and what much it costs. Is there worth it is get he 6 month multiple entry visa of I might only stay 3 months? If I got that visa but ended up wanting to stay longer, can I extend the visa? Or maybe leave the country and come back on a new visa easily?