ALTDEV ALT EXCURSION DUE TO GPWS WARNING DURING MVA RADAR VECTORING.

Narrative

CLOUDY; RAGGED WINTER WX OVER MOST OF NORTHERN CA. SJC ATIS: 4000 FT SCATTERED 650 FT BROKEN 10000 FT OVCST; VISIBILITY 30 MI; WIND 120 DEG AT 10 KTS; LNDG TO S. WE WERE BEING VECTORED ON 270 DEG HDG FOR A NW DOWNWIND TO SJC; AND GIVEN A DSCNT FROM 8000 FT BY BAY APCH (135.2) 'DSND TO 5100 FT MINIMUM VECTORING ALT IN THIS SECTOR.' FO WAS FLYING AND MADE A NORMAL DSCNT FROM 8000 FT; WITH 5100 FT SET CORRECTLY AND VERIFIED BY BOTH OF US. IFR; AND BTWN 5500-5100 FT; WE RECEIVED A CONTINUOUS GPWS OF TERRAIN; TERRAIN; AND IMMEDIATELY CLBED ABOVE THE CLOUDS TO APPROX 6500 FT; EXPLAINING THE WARNING TO APCH. AT APPROX THE SAME TIME AS THE WARNING I NOTICED MY ADI R ALT KICK IN AT 2500 FT AND IMMEDIATELY WENT TO 2000 FT; 1800 FT; 1500 FT. POSSIBLY THE FAST RESPONSE OF THE R ALT TRIGGERED THE GPWS; BUT AT THE TIME WE WERE NOT ABOUT TO ANALYZE THIS WARNING. ON THE GND I CALLED BAY APCH AND TALKED TO MR. X. I STRONGLY RECOMMENDED THAT APCH CTL NOT USE MINIMUM VECTORING ALTS IN IFR IN KNOWN TERRAIN AREAS. EXPEDIENCY; CONVENIENCE; TFC FLOW; ETC PLAYS A PART IN THE SCHEME OF THINGS IN USING MVA; BUT GPWS CLBS ARE UNCOMFORTABLE FOR PAX AND UNNEEDED EXCITEMENT FOR THE CREW AS WELL AS UNSAFE. ALSO IN A GPWS CLB WE DON'T HAVE TIME TO ANALYZE TFC ABOVE; TCASII OR NO TCASII.