First of all, this thread is going in two directions, steam or blow-by ... two entirely different critters. You need to make sure which one it is first. If it's moisture vapors then you have an internal coolant leak. If it's blow-by (combustion gases blowing by the rings/pistions into the crankcase) you could have unseated rings, busted ring, and/or burnt/broken ring land on a piston.
Get a small make-up mirror from mama and put it in the fridge for a while. After the car's warmed up, running and making the "vapor" put the mirror in the vapor stream. If moisture condenses on the mirror then it's coolant, if not then more than likely blow-by.
Once you know that then you can figure (with our help) where to go from there.
Also, wipe your finger inside the rusting tank. Taste the residue (not much though, don't want to get sick from glycol poisening) If you're running any
antifreeze you will be able to tell as it has a distinctive taste if it's an internal coolant leak.