It rains a lot in Vietnam. A LOT. The wet season lasts from May to October in the south, and the best times for traveling to Ho Chi Minh City are late November through January. Consider that "dry season" is a relative term in Indochina. Temperatures range from hot in the winter to hotter in the summer, and the humidity nears 100 about every day. Expect tropical storms often in the summer.

That said, the Vietnamese take little notice of bad weather. Nothing stops for rain (though you might want to sometimes--road conditions are still pretty poor in some places and travel by bus or motorbike can be extremely dangerous in wet conditions).  Air conditioning is standard throughout the city so you needn't worry too much about the heat. 

 The biggest national festival in Vietnam occurs in late January or early February and is called Tet.  This is an excellent time to vist the country, as the streets erupt with color and... well... festivities. Tet lasts about a week (or, for some Vietnamese party animals, up to a month) and is scheduled around the lunar calendar, so you'll have to check for this year's dates.

 For the budget-conscious travellers, TET is not the time to travel to VN since most items are more expensive than usual.