Air carrier departure from LAS experienced TCAS RA with general aviation traffic at 12;000 FT; Reporter alleges ATC failed to provide adequate planning/spacing to prevent the conflict; no FAA record filed.

Narrative

Climbout on the Cowboy departure; assigned radar vectors heading 090; climb to FL190. Vectored at about 8;000 FT to heading 120; maintain 250 KTS. At about 10;000 FT we received a TCAS TA. Located the threat and observed on TCAS at 12;000 FT level; neither one of us could get a visual. At approximately 11;500; we got a RA and the Captain (pilot flying) responded with a nose down pushover. I think I saw the TCAS target at +200. We descended about 500 FT and got a 'monitor vertical speed'. During the maneuver ATC (Las Departure) transmitted 'maintain present altitude; Cirrus at 12K'. I responded we were already in a RA descent. We then continued the climb and advised ATC of the near miss. The Controller was very busy. However; in hindsight; the vector and airspeed assignment were bad and assured a conflict. The Controller never issued an alert or guidance until too late. We never got a visual on the Cirrus; due to its size and the sun position. The Cirrus never changed altitude; apparently never seeing us either. Our plots were merged on the TCAS. Had we not performed evasive maneuvers per the RA; I'm afraid the outcome might have been very bad. The Controller issued a bad vector and failed to monitor traffic.