Most flights into Vanuatu are to the capital Port Vila. It is another 50min flight to the island of Tanna to see Mt Yasur, the active volcano. There are other inactive volcanos on other islands too.
There is no evening walking tour to the volcano that I know of. If you are staying on the airport side of the island (where there are a few resorts with power and western facilities such as Tanna Evergreen) it is a 1.5hr drive through jungle tracks, across ash plains, river beds, steep roads with steam fumeroles at the sides etc to get to a remote carpark with a quite steep path about 100m long, to get to the rim of the volcano. If you are staying in a bungalow near the volcano (probably limited power and no hot water if you need this) you would still possibly be driven up to the carpark. You can walk in either direction around the rim depending on which way the wind is blowing to avoid the smoke or ash but to the right from the top of the path gives a good view of both places it erupts from. It gets very cold up there once the sun goes down. Its best to wear long pants and a jacket and closed in shoes (not shorts and a singlet and thongs like I saw one silly girl wearing...she didn't last long!). Your accommodation will provide a torch for you to navigate the path on the way back down (you will go up in daylight to see it as the sun goes down and it is amazing and seems more active after dark).
Day time temps in Port Vila would be in the high 20's but very humid so it feels hotter with the temp dropping a few degrees at night. You will be at the tail end of the wet season but that shouldn't be a problem. It can rain for a 5 minutes, 5 hrs or 5 days and then the sun comes out. If you are lucky it will just rain a bit at night to cool things off. It can be a couple of degrees cooler on Tanna.
Here is a link to the Port Vila webcam so you can get an idea of the weather til you arrive. http://www.telsatbb.vu/public_cams.aspx
Have fun researching