This is a simple idea to create the fake heat distortion. The purpose is to distort the background image according to the flame/fire motion.
To get the Rest position, simply bring the "rest", "rest2", and "rest2_ratio" into the "mix" VOP, which is, "lerp( v@rest, v@rest2, f@rest2_ratio )" as the VEX. By using "to ndc", we can convert the rest position into remapped camera space, which is, UV. With this UV, we can fetch the color from plate, or, we can export it directly to Nuke or any other compositing software, to do the distortion.
This gives us additional directional distorting than just using the reflection or refraction.