Step 1: Select your object.
step 2: Add a Subdivide or Multiresolution modifier. Vertex painting gets a better result when you have more faces to work with.
Step 3: Switch over to vertex paint mode. And add some colour, the values from black to white will control the mix of the shaders. The darker the shade, the brighter the heat glow.
Step 4: Use this node setup to use the vertex colours you have just painted on.
Adjust the settings to fit your needs.