Night vision will have trouble with fog just like using a spotlight with your naked eyes.
Thermal will cut thru fog and rain, but rain makes it harder to see the ground and brush/weeds as the moisture cools everything off to the same temp. Critters will usually stand out as they are hotter. Under certain circumstances I have had the thermal not be able to see the ground and background due to moisture suspended in the air but not visible fog. With PVS-14 night vision and IR light it looked as clear as daytime, with the thermal only the critters could be seen, about 20min later all cleared up . It was just like when it gets foggy then the dew starts to form and everything clears up. But it only affected the thermal. I think it was water droplets that just happened to be a certain size to scatter the LWIR wavelength the thermals need to make an image. So the critters that gave off a strong amount of LWIR they could be seen but the background and the brush as you looked further away it was washed out as the LWIR was blocked. It happened in a low spot with brush blocking any light wind. The field adjacent was very clear with the thermal. So it's not something you hear about much. I have hundreds of hours behind the pvs-14 and different thermals an this has only happened a few times.