The pre-flood vapour canopy was created on day 2 of Creation when God divided the waters from the waters.
How much water are we talking about? The Earth has water vapour in the atmosphere as we speak, so how is this vapour canopy different than the canopy we have right now?
Evolutionists will rubbish it as it plays havoc with their dating methods.
How does vapour in the atmosphere cause 40K (or any radioisotope for that matter) to change it's rate of decay?
This canopy protected the earth from harmful radiation and greatly enhanced health by reducing mutation rates.
Radiation is needed by plants to produce sugars through photosynthesis.
The oxygen content was also higher than today, which was very beneficial to health,
Evidence please. From what I have read higher levels of oxygen can have serious side effects.
The canopy would have given a uniformly temperate climate around the earth,
How? The density of sunlight energy is not the same across the globe due to the Earth being round. Water vapour at the equator would necessarily be warmer than water vapour at the poles due to this difference in energy density. The result would be the same as now. You don't need to look any farther than Jupiter to see the results of a gaseous and thick atmosphere. Ever seen the red spot on Jupiter?
There was no volcanic activity till after the flood when the earths crust was broken up, originally it was all one land mass.
Evidence?