Thanks .... he actually understood my jibberish of a post.As had been stated, no matter what the internal logic is, the Avatar's abilities only make him immune to Melta and Flamer weapons, because that is what the rules as written say. Even though being immune to heat SHOULD also make him immune to Laser and Plasma (which do largely heat based damage), nowhere does it say he is, so he is not.
Likewise, nowhere does it say that an exploding vehicle does damage as a Melta or Flame weapon, so he is not immune.
In reality, the major source of damage in an explosion like that would be the shockwave of the blast itself (not to mention all the flying shrapnel it makes) Just the sheer force of the blast would tear most things to pieces before the fire even catches up to them, if there even is a fire.
Hollywood explosions aside, a lot of explosions are relatively heat and flame free. Even in a fueled up, ammo-caryring military vehicle, any fire would likely be an afterthought to the blast itself. Whatever's left after being blown to hell might catch on fire, but that wouldn't be the real damage dealer.