Air carrier flight crew reacted to a close encounter with a helicopter.

Narrative

On vectors BOUNO4 arrival at 2000 FT about 12 NM from HPN; the First Officer was pilot flying. We were slowing from 250 KTS and watching a target approach on TCAS 300 FT below us. NY approach issued 180 KTS and a traffic advisory; 10-11 o'clock and 3 miles and closing. I requested a climb of about 300'; but the controller only suggested we turn to fly behind traffic. We were rapidly closing on traffic when I acquired visual on it; I believe it was a helicopter. Since the First Officer had not acquired it; and I perceived it to be a collision threat; I took the controls and initiated a 30-35 degree bank turn to the left. I had been focusing on avoiding the traffic; but in the turn I glanced at the instruments and realized that with the power at flight idle for the rapid slowing from 250 to 180 KTS; combined with the sudden increase in load factor; the stall tape was rapidly approaching. I immediately added power; concurrent with receiving a TCAS RA to monitor vertical speed. I believe we came within 15 KTS of stall speed. We recovered and remained 2-300 FT above the traffic; then continued a normal approach. Multiple complications and unfortunate timing. The First Officer had not yet called for Flaps 8 during the deceleration. When I took the controls I was primarily focused on avoiding the traffic; and it took me a couple seconds to realize the power and flap configuration needed adjustment. Compounding the problem was my mindset to comply with our ATC clearance (altitude); especially since the controller specifically did not authorize the climb I requested; combined with my concern with the congested NY airspace. I should have used my emergency authority to climb anyway; and I should have allowed the First Officer to remain pilot flying.