You are going to Vietnam for bowls of phở at dawn, crispy bánh xèo in alleyways, and iced coffee on plastic stools. The trick to keeping it comfortable is timing your trip for the shoulder months of March or September when the weather is decent and crowds thin out.
A typical trip runs about $2500 once you factor in flights, places to sleep, and endless bowls of noodles. Spreading that out over time takes some planning so you are not hit with one massive bill right after you get back.
That intro window is a deadline, not free money, and if you still have a balance left when the 18 months run out, standard interest charges will kick in on what is left over.