When in Port Vila, this is definitely not-to-be-missed. The best time to go would be the morning when there would be the least amount of visitors.
The hike to the waterfall takes about 10-15 minutes and you're free to go at your own pace. It is quite a pleasant hike and the paths are well laid-out. When you reach the waterfall, make sure you climb to the top, which is the best spot. The water is cool, fresh, perfect to go for a swim on a hot day.
Alternatively, the most cost-effective way to get here would be by a tour. There are two departure times, at 9am or 2pm, whereby a shuttle picks you up from and then drops you off at your hotel. The duration is two and a half hours and it costs 2,500 Vatu, which is approximately $25. You can also go there solo, by bus or taxi.