LTT DSNDS FROM ASSIGNED ALT BECAUSE OF A PERCEIVED CONFLICT.

Narrative

WHILE WE WERE BEING VECTORED ON THE ABOVE STATED NIGHT; TO EWR RWY 22L; WE WERE ON AN ASSIGNED HDG OF APPROX 260 DEGS. NY APCH INSTRUCTED US TO TURN L TO A HDG OF 100 DEGS. WHEN I LOOKED TO CLR THE AREA; I WAS CONFRONTED WITH WHAT APPEARED TO BE A LARGE ACFT AT OUR ALT HDG DIRECTLY FOR US. I INSTRUCTED MY FO TO GET IMMEDIATE VERIFICATION AS TO WHAT THIS ACFT WAS DOING AT OUR ALT. MY FO TRIED TO CONTACT APCH BUT WAS BLOCKED BY ANOTHER RADIO CALL. AT THIS TIME; I BEGAN A DSCNT TO AVOID THE IMMINENT COLLISION THAT WE WERE CONFRONTED BY. THE APCHING ACFT APPEARED TO BE LESS THAN A QUARTER MI AND CLOSING FAST; AT THE TIME OF THE ALTDEV. THE TOTAL DEV WAS APPROX 400 FT. THROUGHOUT THE ENTIRE SIT; WE WERE IN AND OUT OF A RAGGED THIN LAYER OF CLOUDS WITH TOPS OF APPROX 3500 FT. MY FO IMMEDIATELY TRIED AGAIN TO GET CLARIFICATION AS TO WHAT THE ACFT WAS DOING. THE CTLR TOLD US NO OTHER ACFT WAS WITHIN 2000 FT OF OUR ALT. AT THIS TIME; WE WERE ABLE TO DETERMINE WHAT APPEARED TO BE AN ACFT APCHING US WAS; IN REALITY; THE LIGHTS OF 2 ACFT DIVERGING WHILE BEING VECTORED INTO EITHER JFK OR LGA. WE IMMEDIATELY ASCENDED BACK TO OUR ASSIGNED ALT. THE ENTIRE DEV TOOK PLACE IN APPROX 30 SECONDS. I ELECTED TO DSND BECAUSE WE COULD NOT GET IMMEDIATE VERIFICATION AS TO WHAT THE OTHER ACFT WAS DOING AT OUR ALT. ONE OF MY INITIAL THOUGHTS WAS THAT THE OTHER ACFT; WHICH APPEARED TO BE A LARGE OR HVY ACFT SHOULD BE PICKING US UP ON TCASII. MY AFTERTHOUGHT AS TO HOW THIS SIT COULD HAVE BEEN PREVENTED IS THAT ALL ACFT USED IN ACR OP SHOULD BE EQUIPPED WITH TCASII. THIS WOULD HAVE PROVIDED ME WITH A MEANS TO MAKE AN IMMEDIATE DETERMINATION AS TO WHETHER THE APPARENT ACFT PRESENTED ANY DANGER TO THE ACFT I WAS FLYING. THE COMPANY I FLY FOR IS MAKING PROVISIONS TO INSTALL TCASII ON ALL OUR ACFT IN TIME TO COMPLY WITH THE FARS. I WOULD LIKE TO EMPHASIZE THAT THE ILLUSION THAT WE SAW WAS EXTREMELY REALISTIC LOOKING; AND THAT MY ACTIONS IN THE FUTURE WOULD BE THE SAME.