In Vietnam, it is not easy to define what will be the best time to visit because broadly speaking, the weather in Vietnam is dictated by two seasons -- the southwest monsoon from April to September and the northeast monsoon from October to late March or early April.
People often equate monsoons with rain, but that is only partly the case in Vietnam, as a number of regional variations affect the rain. What is worth remembering though is the southwest monsoon is warm and wet and the northeast monsoon cool and not so wet.
Generally speaking, destinations in the north such as Hanoi and Sapa are great in October, November and December, as you'll see little rain and should have clear skies and temperate conditions.
The coastal stretch from Hue down to Nha Trang is great in the first half of the year, from January through to July, while Saigon and the Mekong Delta are best from November through to February or March.
For how many days that fit for travel across Vietnam whilst covering most major landmarks, I could say that you should give yourself with the minimum of 14 days for holiday (that is given in term that you are able to take flight). Since you are retired in your seventies, I am strongly suggest you to arrange a personal tour rather than do it on your own. Let us plan for you and take care of your hassle.
