There is no real best or worst time to visit Vietnam, with travel possible all year, but the seasons vary across the country and you should be prepared to encounter some rain whenever you decide to travel.
Vietnam has become even more unreliable in recent years as a result of global warming. The winter months of 2007/08 were especially harsh, with some of the worst weather in the region for over 40 years. Serious flooding affected Hoi An in November, and Hanoi and the north was colder in January than much of the UK.
Northern Vietnam, (e.g. Hanoi, Halong Bay and Sapa) can be quite cold, damp and misty from November to March. However, it is also possible to experience lovely sunny, cloudless days here during this period. You might need warm clothing and an anorak or other waterproof in case of rain, but these can be cheaply purchased locally if required.
The central region (Hué, Hoi An, Danang and Nha Trang) is at its wettest from September through to December. Traditionally this period sees monsoon rains and occasional typhoons which can cause flooding in the area. Whilst this is rarely of such a level as to seriously affect an itinerary we do advise people travelling in this area over this period that there may be last minute changes to itineraries to accommodate for the weather conditions. Our ground agents in Vietnam are extremely adept at monitoring events and adapting itineraries accordingly.
Some people might say that our summer months (June, July, August) are the best time to visit Vietnam. Unusually for Asia the weather in central Vietnam is likely to be sunny and clear during this time, and although there may be short downpours in the south and north, many people see no rain at all and enjoy cloudless sunny skies. Southern Vietnam will be very humid, and thunderstorms cannot be ruled out, but these generally clear the air and do not prevent you from sightseeing. It is just worth bearing in mind if considering a beach break.