The vapour barrier should be on the inside of the insulation, I believe, to stop internally generated water from condensing into the insulation, when it reaches a suitable point in the temperature gradient.
As above - the roof is where the warm (moist) air will congregate.
I suspect that, if you are just breathing in there and not boiling kettles or using unflued combustion heaters, then you may well be OK as it stands.
The vents, for a reasonable bit of air-change, will definitely help.