I was there about 2 weeks ago for a cousins wedding. If you have money and want to live the typical western Fiji vacation, you'll be spending all your time on day cruises to small private islands and/or staying in
Denarau. Denarau has some pretty decent nightlife I guess... it has a Hard Rock Cafe that has a live band and etc, etc. But yeah, Denarau is your standard tropical resort location with a bunch of fancy hotels and bars and swimming pools.
Actual city life is very different. The south island was flooded pretty badly so Nadi town has been kind of beat up but it's the island to go to. It's still functioning perfectly fine as a town, but it's not on the more glamorous side as it used to be. I didn't get a chance to go to Suva, but that's the big major city. There's probably quite a bit to do there, but if you stay in/near Nadi, there are some bars and clubs you could go to. Ed's Bar comes to mind, as it was a popular one that came up when we considered where to go at night. There was another night club, Ice or Frozen or something like that. And then there's bounty which is another bar. Honestly, they're all on the same street just outside of Nadi town, if you find one, you've found them all lol.
Honestly, it really depends on what kind of vacation you're wanting. If you want the sheltered, luxurious vacation, it's more than easily obtainable. And if you want a vacation more rooted in the country and where the natives actually live, well, that's very doable as well, and I'd definitely recommend that more anyway. The locals are all really friendly too, and while yeah, you do have to be cautious as with anywhere you go, if you want to get to do all the really good, non-western-typical stuff, you gotta talk to the locals.