The CDC recommends Hep A Hep B and Typhoid. There was an outbreak of Typhoid in a remote province of Viti Levu in June 2010. No tourists were affected. The wet season is November to April and most of the country outside the major towns is NOT connected to sewerage - say no more. Stick to bottled water in Fiji (the ubiquitous Fiji Water).
Great advice on the Dengue Fever from Suzi. take repellent.
In the highly unlikely event you get typhoid and have not been vaccinated, your insurer MAY argue you contributed to your illness and could deny your claim - and if it involved extensive medical treatment or medical evacuation that could be expensive. Get the jabs.
We have more info on Fiji on the WorldNomads safety hub:
http://journals.worldnomads.com/safetyhub/story/62763/Fiji/Drinking-Fiji-Water about 6 years ago