The reasoning is that you've created a 2nd vapor barrier that creates moisture that has nowhere to go other than into the roof panels and as a consequence gives you rot which will have to be dealt with 10-15 years down the road.

I would recommend an underpayment with vapor barrier just to give it a little bounce. 

I just put in $20,000 new sliding, $5,000 new electric box and wirings, carpet, water heater, new paints, new fridge, new vapor barrier and insulations.

Then they will inspect framing, insulation/Vapor barrier , amp final inspection.