You're partially correct and definitely on the right track. However what is happening when the manifold is cold is the fuel is atomized as it is pulled from the carburetor by the air passing through but when it gets to the cold intake manifold and has to do a 90 degree turn it slams into the manifold and condenses from the cold and puddles up there.
The exhaust manifold is far more effective at combating this because it heats the intake much faster than waiting for water to heat up while the pooled up fuel hurts your emissions and sneaks past your rings when it gets to the combustion in liquid form thereby washing oil off the cylinder walls and contaminating the oil in the crankcase.
Vapor locking, by definition, does not occur to cold fuel. It is a result of hot fuel boiling and turning to "vapor". I'm not sure what you are experiencing when your truck is cold but it is not vapor lock. Perhaps you are experiencing an overly rich mixture caused by what I explained before.
If you simply must use a header (like I did before I knew better) the liquid heaters are better than nothing but it will never be as effective as a manifold and you WILL see increased wear from the overly rich condition before the water gets warm enough to have an effect. It may be academic but a fact just the same.
I've got some Yotas, some bikes and a drag car...