If the air in the crater is rather cool, then I believe Twelfth's answer is correct. If the crater is full of hot air (I assume this because you mentioned the creatures "have learned to ignore the heat of the air") I don't believe that this will work. First of all, what you're describing sounds like an inversion, but those occur when their air is colder than the air above it. If you have a hot environment the air is going to rise and mix with the air above it. I believe the convective forces that emerge would cause the crater to be rather windy, with lots of updrafts and downdrafts. That would most likely clear out any water vapor that would form.
Also, fog would have trouble forming in such an environment. According to Wikipedia,
Fog forms when the difference between air temperature and dew point is generally less than 2.5 °C or 4 °F.
The dew point is also affected by temperature and pressure - higher temperature and lower pressure (which you'd have here) increases the dew point, making it harder to have fog. If the humidity is high enough you could still have fog, but that introduces its own problems - an environment that is very hot and very humid would be unbearable for everything but extremophiles, and those are limited to micro-organisms. If a person matched their body temperature to that of the air to be invisible to IR vision, they'd be dead.
As Twelfth mentioned, infrared vision wouldn't work very well through fog/steam, even in the hot environment scenario.
Unfortunately tunnel vision wouldn't work either. In heavy fog you can't see things because the light bouncing off of the objects gets scattered before it can reach you. Nothing passive will allow you to "pierce the fog" because there's simply no organized light that reaches you.
One other thing that might not work for your purposes would be to change the composition of air in the crater. If you had heavier elements in the air it would be more dense, making an inversion more likely and also reducing the dew point and making fog possible without killing people from the heat. Unfortunately the denser air would also push out oxygen (and probably be toxic anyway), so people who visit would have to bring oxygen with them. This also makes it less plausible for any non-microscopic creatures to naturally develop to live there as it would be a large evolutionary leap to handle air like that.
Obviously anything can happen with a sufficient amount of handwavium, anything can be possible. So how do we get this environment with minimal effort?
One of the key points of inversions is that the air below becomes more dense than the air above. So what if the area is inherently more magical, and that causes the air to be more dense? This solves your problem in the same way that the non-magical denser-air solution does without killing people from asphyxiation. Additionally, if we already have a strong magical field there why not replace IR vision with vision that relies on the ambient magical field? It's easy to believe that fog wouldn't obstruct the magical field, and a person trying to match the ambient magical field would be less dead than someone who tries to match the ambient (hot) temperature. Also, the creature's tunnel vision could be explained by requiring the creature to be able to concentrate to see the field clearly - without concentration they can see the general shape of the field, allowing them to navigate without stumbling, but they can only see subtle changes in the field if they are concentrating in a particular direction.
I'm not sure how much more dense the air would need to be in order to make this work, but based on some rough calculations I did I think it would only need to be somewhere around a 30% increase in density. For being on a mountain top, that means you'd have around the same density as if you were at sea level. That could also give your characters an added reason to go into the crater - after hiking through the thinner air to get there, they find a place where they feel like they can really breathe again.