Hi and thanks fo using JustAnswer.com.
Obviously you still have moisture passing through the mortar at this point and leaching the effloresce to the inside surface. Short of waterproofing the exterior the only other thing with might solve your effloresce would be a parge coat of mortar. Clean the existing brick and mortar with acid first and rinse.
Another option is move the stairways inside wall away from the outside wall to get the width you need to do the foam/vaporbarrier/wallboard.
I hope that this information was helpful to you. If it was please remember to click "ACCEPT" on your screen to make sure that I am paid for my efforts. By clicking ACCEPT you are not giving up your ability to ask more questions pertaining to this subject, and I will be happy to respond to these as well. Please take a moment and leave feedback, it is very important!
Thanks for your accept!
this is the information I received regarding your accept
Customer paid $15 on 10/12/2009 at 9:15 AM
I do not see any other charge