An air carrier Captain reported an NMAC with a small aircraft on approach to CVG; and was critical of ATC handling.

Narrative

While descending into CVG we were cleared to 3;000 FT. Later our clearance was amended to 3;500 FT due to a VFR aircraft 'popping up'. Controller issued vectors to base which put us head on to VFR aircraft at 2;900 FT-3;000 FT crossing ILS 18L final approach path. We were IMC at 3;500 FT. Using TCAS both pilots observed the target at 500 below our altitude. TCAS changed to yellow as we approached target head on. TCAS showed VFR target climbing to -300 FT which changed target to RED and caused RA CLIMB status. I disconnected the autopilot and climbed to around 4;300 FT to miss the climbing target. At this point we were clear of the conflict and nearly on the localizer. Controller issued clearance to join localizer and descend to 3;000 FT again. Some where around 3;000 FT we descended through the ceiling and visually navigated to landing on 18L. The event occurred because of an aircraft operating below the Class B airspace (2;900 FT) at the same position where IFR arrivals would normally be descending to a similar altitude while tracking the ILS and then they began climbing into us. The Approach Controller was pressing for us to accept a visual approach clearance due to a proceeding aircraft having the airport in sight above 3;000 FT to the west of CVG. The radar vectors were issued to keep us in close for a possible visual approach. Unfortunately we didn't break through the ceiling until below 3;000 FT. When a known target exists on the radar; which will cross the IFR approach course; please don't vector an aircraft head on to them at similar altitudes (3;500 FT IFR/3;000 FT VFR). I really wished that we would be given a longer downwind vector or a base vector that at least didn't cause us to converge on the same point in IMC conditions.