I guess I would need some more information about the job. If it's on cured concrete, and the moisture is still high, there is probably an issue that's causing the moisture that should be addressed. I had one commercial job that did not have a vapor barrier under the slab (this may or may not have been the problem, but the moisture was high everywhere and it was a huge job, so a leak was very unlikely) and we couldn't wait any longer for the moisture to dissipate so the contractor agreed to pay for a very expensive sealer that had to be rolled onto the floor and allowed to dry completely, prior to installing the floor covering. Other than this, I can't think of a situation where a vapor barrier would be necessary.