My issue with vapor-phase is that it tends to cause a lot of tombstoned or sleeping components. It's pretty good for multi-leg components like power ICs and processors and BGA components, but not so good on two-conductor resistors, capacitors, jumpers, etc. This is mostly because with vapor-phase you can't really control the thermal profile of the reflow.
My understanding is that Galden vapor is dense, so you get a layer of hot vapor near the fluid, with lower temperature further from fluid (where there is less vapor)- the required thermal profile is maintained by controlling position of the boards..
