You will actually not want to use a vapor barrier but a drainage plane to stop bulk water from entering the wall cavity.
- You will need to replace your vapor retarder with something like housewrap or felt paper.
- Either one will do the job.
There are numerous housewraps on the market that can be purchased from any lumber company or home improvement outlet (you want the ones without holes).
A good drainage plane allows water in the form of vapor to go through the material so that the wall can dry out should water somehow enter the cavity (but the drainage plane should stop bulk water from entering the wall assembly).
Your best choice will be to use #15 or #30 felt paper because you can buy it in smaller quantities than you can for housewrap.
For more information:
Read "What is the most efficient wall assembly for new construction? I'm having trouble talking my contractor into 2x6, OSB, and house wrap." a Q&A answered by Elizabeth DiSalvo.