I saw all I really needed to see in 14 days. You needs about 3 days in cambodia for ankor watt and travel times on bus, then get outa there unless you are living or doing work the entire country has been destroyed ecologically, human wise, economy wise, by the french, a genocide, fammine, us bombs and land mines, and no china choppin down the last of their rain forest. Although I here but have not seen that there are some beautiful untouched beach locations in the south, but vietnam will quench your beach thirst, North to south, south to north doesnt matter,
Hit up Ho chi minh, and see some museums, eat some big bowls of soup and crawl in the cuchi tunnels, the stop in Hoi An and soak up the sun on a crazy beach, go out for beautiful meals, stay in a nice 10$ a night hotel ( trust me its worth to splurge this time) and get some custom made clothes made up for ya! Then hit up Hanoi for some hustle bussle colour and good times or bizare times. Like is said North to south, south to north doesnt matter.
ENJOY! I LOVE VIETNAM