Vanuatu is a remarkably adventurous totally unique place on many levels, including the diving. There is a recent thread on it here on SB. I love and live on Hawaii but would pick Vanuatu (or Fiji or Palau) over it any day unless you're needing/wanting a packaged typical tourist experience in a First World country with all the medical, travel and infrastructure implications.
Having said that there is good diving in Hawaii and occasionally great animal experiences but the reefs are impoverished compared to Fiji or Palau or Indonesia (Komodo blows it away). Vanuatu's reefs are better than Hawaii but not as good as other places like Fiji but it also has the Coolidge, Million Dollar Pt, fresh water Blue Holes, an accessible amazing erupting volcano, an amazing diversity of tribal culture and a choice of remote adventure or developed "spoiled" civilization (sometimes a bit of both simultaneously.) Vanuatu is a new nation, rough around the edges to say the least, comprised of many tribes and expats and developing in fits and starts. Australia and New Zealand are helping out. The diving operations seem to be all run by Australians and are quite professional imho. Guides are usually locals and they're great too so I always felt safe diving, You absolutely need guides to dive the immense Coolidge.
I'm diving here on the Big Island regularly (in the next 8 hours) and enjoy it greatly. It's a beautiful place. Very convenient from the US mainland and has great reliable flight connections with Japan/Asia. (Scoot airlines ) I have encountered whale sharks, Mantas (of course) hammerheads, lots of dolphins, tons of eels and nice fish life. However, that is from tons of diving and except for the common mantas you might or might not see those things on vacation. (Whale Sharks are very unusual,) Moreover, Hawaii is biologically isolated by vast distance and is too far from the coral triangle (or any other reef systems) and a bit too far north to have Fiji level diversity. We have only a few coral species, for example, and they're beat up in many places. Fish species are far less diverse than Fiji etc.- larval stages often can't make the 2500 mile journey so they never got to Hawaii.
my 2 cents.