Rookie,
Is this your first frame? If it is, you may be attempting to run before you've learned to walk. (Based on the questions you asked)

You could use 1 1/2"x 6 T/G. Not sure why you need vapor barrier.
Most barns I have seen are Common Rafter with planks spaced for air circulation and cedar shingles. Eventually recovered with steel at some point.
Another solution would be to double the purlins(24" centers) and apply the steel directly.
Also, depending on the roof pitch, the 4x6x12' purlins @ 48" centers seem a bit light. (A gut feeling) At 24" centers, they would be better. To be safe, crunch all the numbers for load and wood species before you finalize your design. If you have any doubts, ask an Engineer to approve you design.

Just my two cents worth...
Smart remarks are not required guys.