The narrow S-curve of coastal Vietnam makes itinerary planning quite simple: You're going either south to north or north to south. Hearty backpackers take at least a month (if not more) going by "open tour bus" and/or other local transport, while jet-hopping tour groups cover the highlights in as little as a week to 10 days.
If you have just 4 or 5 days, base yourself out of either Hanoi or Ho Chi Minh City. With any itinerary, try to give yourself a good amount of flextime for days of rest at a beach, tummy trouble, or chance meetings, side trips, and inspirations along the way.
Some international flights land in Danang, and some travelers cross over land from Laos to Vinh or Hue -- which are midway along the coast -- or to Ho Chi Minh City from Phnom Penh, Cambodia. But most people arrive in Vietnam by air into Hanoi or Ho Chi Minh City. Most tours of Vietnam start off in one of these two busy cities. First-timers are often overwhelmed by the chaotic traffic, busy markets, pesky touts, and hot weather -- hang in there, it just takes a bit of getting used to.
The most important thing to consider is weather. The country's opposing monsoonal systems in the north and south may sound a bit complicated, but here's what you need to know, in a nutshell:
The south, including Ho Chi Minh City (Saigon) and the Mekong Delta, is always hot, with a rainy season (ranging from afternoon thunderstorms to all-day downpours) from May to October. The Central Highlands is best during their "somewhat" dry season from January to May. Along the coast in central Vietnam, the monsoon season is inverted, and though the temperature is steady and warm, you can experience rain from October to February (and some tropical storms in between). The north is much cooler, with a wet and cold winter from December through February (quite cold in the mountains, and there's even snow), and a very hot summer with rains (best time to visit is in the fall, Sept-Oct). You can catch high season in multiple regions if you visit on the cusp of seasons: For example, travel in the north in November before you head south.
Important note: If you're landing directly in Vietnam after a long-haul flight from Europe or the U.S., know that jet lag is a formidable foe. You might hit the ground running on your first day and then feel exhausted on day 2 or 3 -- especially due to the heat and chaos of urban Vietnam. Try to get onto Vietnamese time immediately (go to bed at a reasonable hour even if you're not tired, and wake up in the morning); some people also say that taking melatonin, an herbal supplement, helps with jet lag. All of the itineraries assume that you're rested and ready to go, so you might want to factor in some downtime at the start of your trip to get rested.