Will you have your own boat and other diving equipment? From what I remember Vitu/Witu doesn't have any dive ops on the islands.
There are land based resorts around the main dive areas.
For land based, Port Moresby is underrated as a dive destination. It has awesome WWII wrecks and the sea mounts have good life. Nothing in terms of pelagics though. I like doing it from Port Moresby (staying either at the Crown or Airways (depending on my air travel schedule)). I used the Dive Center run by John Miller and his wife. They are fun people and the best part is they left me alone to shoot. They were very helpful with getting the logistics to get the shot. The boat is slow and noisy though.
POM is known for the crime but there was only one attempt of robbery on me in Mt Hagen. An alternative to staying in POM is Loloata, an island resort just off POM. I've stayed there a couple of times but preferred staying in the city.
There are resorts like Tawali which is in Milne Bay and has some very nice diving and probably my favorite macro area in PNG. Tufi is also very nice. Madang I thought wasn't as good as Tufi but it was different. Then there's Kavieng, which has more pelagics but the dive sites are far from the resorts. Same for Kimbe Bay's Walindi resort.
If it were my first time in PNG, I'd do Milne Bay for land based. Prolific life and variety and the dive sites aren't that far apart unless you want to hit the pelagic sites.