B737 had a conflict with a VFR Hawker business jet over a major arrival fix.

Narrative

A Hawker business jet flying VFR requested approval from Washington ARTCC to fly over VCN while we were enroute to that fix. Washington ARTCC told the Hawker that there were numerous aircraft at and around VCN. Washington pointed us out to the Hawker; who said he had us in sight. We were cleared direct to VCN; and were instructed to descend to cross VCN at 8;000 MSL; and to enter holding there as depicted. Based on the discussion between the Hawker and Center; we were looking for; and saw the Hawker a few miles from VCN. When we first saw him; he was at 9;500; which was 500 FT above us; and he was descending. As we continued toward VCN; it became apparent a conflict was imminent; and the TCAS TA sounded. The Hawker continued to descend; and descended faster than us so that as we approached VCN we were at the same altitude. As we started to discuss asking for a turn to avoid the conflict; the TCAS RA told us to 'monitor vertical speed'; commanding no significant descent. A few seconds later; it told us to climb; commanding a 1;800 FT per minute climb; which we complied with; and told the controller we were doing; while watching the Hawker visually. The Hawker passed below our nose and in front of us. I estimate the closest approach was 800 FT. vertically and 200 FT. horizontally. If we had not taken evasive action; we would have closed to within 300 FT. slant range from the Hawker. We never saw the Hawker make any change to his flight path. After passing; we returned to 8;000 FT. and entered the holding pattern at VCN.