What would the van der Waals parameters look like for gaseous metals?
Aside from the metal vapor oxidizing, the pressure of gas over your weld is unlikely to be much different from one atmosphere whereas the critical pressure of almost everything is 10 atmospheres or (often a lot) more.  Generally gases become non-ideal when the pressure is a large fraction of the critical one; we are well below pressures where that occurs.  So the Ideal Gas Law works -- until the metal vapor oxidizes.