They would be broken down to individual water molecules. Unless the hydrogen bonds are broken, they should still be able to reform whenever the water molecules have a chance to escape. Technically, If they stay in vapor state, they should be able to live indefinitely. But now we're really grasping at straws and speculating. We're at Ancient Aliens level of speculation now.
They cannot live indefinitely because this state requires chakra to use; once they run out of chakra in the lava example, presuming they are not forced to revert back sooner, they would return to normal and be melted or die from 0 chakra, whichever came first. As a vapor state, similar principle applied, but recall that the Hozuku are not purely water, they are more like a water-based slime of sorts that would likely have a different boiling point than regular water. The water would be boiled while the leftovers would probably take longer to evaporate, presuming they don't just burn or are turned to ash.
So I think getting hit with scorch release would still kill a Hozuki off, transformed or no, as would being encased in lava.