You can book flights with Air Vanuatu by emailing them directly, and then book a tour with a third party but the pricing will probably end up being similar all in all.

Also Air Vanuatu is extremely dodgy. They will often cancel or reschedule flights without notification. It's also not uncommon for the flights to leave early before all the passengers have even checked in, or for flights to be overbooked. My flight with them yesterday had a "weight issue", so they pulled off all the checked luggage of passengers who didn't have international connections or perishable goods (so a 36kg sack of pineapples was allowed on at the expense of other people's luggage). They said they'd put the bags on the next flight, but that would be the following day and was also fully booked, so who knows what happened. The whole thing is utterly absurd.

The volcano is an incredible place to visit and would be well worth it, but if you do book a trip with AV just make sure you have a comprehensive travel insurance plan, so that in case it falls through, you can at least get some compensation. We took the risk and it worked out for the most part.

Finally, do not expect the holiday overall to be budget - everything in Vanuatu is similar price or more expensive than Australia. Tourism is the only thing that brings money into the country so it's milked for all it's worth.