Not sure you need or want a vapor barrier with cellulose, particularly in a shop.
http://www.regalind.com/techPDF/General%20Information/Is%20%20A%20Vapor%20Barrier%20Needed%20for%20a%20Cellulose%20Insulatied%20Home.pdf
The "P" shaped tread style are safer and easier to use than the 3 stringer type:
http://static.monolithic.com/gallery/homes/clark/pictorial2.html
http://www.staircases.org/spacesaver.htm