A vapor barier is called for in this application. Irregardless of it being an interior wall. Even an interior block wall have a higher moisture coefficient than surrounding timber and dry wall walls. Moisture will rise and leach up the expanse from the base at a higer rate than a non masonary wall. The plaster coating will not be effective as a vapor barrier. Optimumly, it should be removed, but with the work and expense involved, I'd advise against it. I know you probably know this, but leave a one and a half inch space between the plastered wall and your new wall. In addition to the vapor benefits, it will act as an acoustical dead air space. Plus, it will give mice a nice little walkway between walls.
Whatever you do, do not think that heating insulation will be effective as a sound barrier. Use acoustic insulation. The ultimate is multiple layers of shag carpeting between the structural members. I know it sounds silly, but it really works. I've designed and constructed sound studios on a few occasions, and shag carpeting is phenomal stuff for sound deadening.