Map from Google - Vanuatu

In Vanuatu the climate is influenced by the trade winds, and it's equatorial in the northern islands, ie hot and rainy all year round, while it's tropical in the central and southern islands, ie hot and rainy from December to March, and cool with less rainfall from May to October.
The islands are in fact scattered over an axis, oriented from north to south, about 550 miles long; the southernmost islands are located at a latitude of 20 degrees south.
Many of the islands are volcanic, and have inland slopes which enhance precipitation. The rains are most abundant in the northernmost islands, where even 157 inches of rain per year fall (see Vanua Lava), including 8 inches per month even in the least rainy months (which are July and August), and more than 17.7 inches in March (the rainiest). Proceeding south, the rains decrease gradually, down to 87/100 inches in the southern islands, even though in the small and flat islands they go down to 60 inches.
The period from December to March is hot, humid and rainy across the country, with highs around 84/88 °F and lows around 73/77 °F. The rains come in the form of intense downpours, which soon leave room for the sun.
The winter season, from July to September, is warm in the northern islands, with highs around 81/82 °F, as we can see from the average temperatures of Vanua Lava.
Average temperatures - Vanua Lava
Vanua LavaJanFebMarAprMayJunJulAugSepOctNovDec
Vanua LavaJFMAMJJASOND
Min (°F)737373737373727272727373
Max (°F)868686848282818181828284

The rains in Vanua Lava, as in general in the northern islands having an equatorial climate, are abundant throughout the year. Here the average rainfall.
Average precipitation - Vanua Lava
Vanua LavaJanFebMarAprMayJunJulAugSepOctNovDecYear
Vanua LavaJFMAMJJASONDY
Prec. (in)16.314.217.516.714.69.68.99.31215.410.411.8156.7
Days201923201917151517181417214

In these northern islands, the sea is warm all year round, as we can see in the following table.
Sea temperature - Vanua Lava
Vanua LavaJanFebMarAprMayJunJulAugSepOctNovDec
Vanua LavaJFMAMJJASOND
Sea (°F)848484848482818181828284

Moving south, winter becomes gradually cooler; in the capital Port Vila, in the central island of Shefa, the daytime temperature is around 77/79 °F, while in the southern islands it drops to 73/75 °F. At night the average temperature is around 70/72 °F in the northern islands, and around 63/64 °F in the central and southern islands, where it can sometimes even drop to 55/59 °F.
Average temperatures - Port Vila
Port VilaJanFebMarAprMayJunJulAugSepOctNovDec
Port VilaJFMAMJJASOND
Min (°F)727372726864646363646870
Max (°F)868684828179777779818284

In Port Vila, as in general in the central and southern islands, the decrease in rainfall from May to October is evident. Here is the average rainfall.
Average precipitation - Port Vila
Port VilaJanFebMarAprMayJunJulAugSepOctNovDecYear
Port VilaJFMAMJJASONDY
Prec. (in)12.61114.48.35.35.14.34.34.14.36.97.187.8
Days181720171413121212121314174

For swimming, the sea remains quite warm even in the central islands, where the sea temperature drops to 79 °F from June to October.
Sea temperature - Port Vila
Port VilaJanFebMarAprMayJunJulAugSepOctNovDec
Port VilaJFMAMJJASOND
Sea (°F)828484828179797979798182

In the southernmost islands, the temperature becomes even cooler, so that from July to September, the average daily temperature drops to 68 °F, as we can see from the data of Aneityum.
Average temperatures - Aneityum
AneityumJanFebMarAprMayJunJulAugSepOctNovDec
AneityumJFMAMJJASOND
Min (°F)707070686463616161646668
Max (°F)828482817777757575777981

Consequently, in the southernmost islands even the sea becomes cooler, and from July to October the water temperature drops to 75 °F, so it's still warm enough for swimming, as we can see from the data of Aneityum.
Sea temperature - Aneityum
AneityumJanFebMarAprMayJunJulAugSepOctNovDec
AneityumJFMAMJJASOND
Sea (°F)818282817977757575757981

The amount of sunshine in Vanuatu is never great, but it's neither poor: the sun is never missing, but some cloudiness, maybe associated with showers and thunderstorms, is always possible. In Port Vila, even in the least rainy months, from July to October, there are about 12 days with showers and thunderstorms per month.

The Vanuatus are affected by tropical cyclones from November to mid-May, but they are most likely from December to April. Outside of this period, there has been Cyclone Xavier which passed near the northern islands on late October 2006.

Vanuatu

When to go

The best time to visit Vanuatu is from July to September in the northern islands (up to Sanma and Malampa), because it is less hot and rainy; while in the central and southern islands which include the capital, the best time is from May to October. Since from June to September in the central and southern islands it can get cool, you may prefer the period mid-May to mid-June and even more October, when it's a bit warmer and the heavier rains are still to come. Even in the least rainy months, however, there are plenty of showers, especially in the northernmost islands.

What to pack

In winter (June to August): light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, possibly a light jacket for the southern islands; light raincoat or umbrella. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In summer (December to February): tropics-friendly, loose fitting clothing, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella.