An air carrier captain inbound to SJC believes approach control was descending aircraft below safe altitudes for doing so.

Narrative

We were vectored off the JAWWS 2 Arrival to a left downwind to 12R at SJC. We were cleared to descend to 5500 feet West of the Airport over the mountains where the MSA is 5600 feet. I was watching our position closely on the area chart and saw the highest terrain in our area was 4100 feet. The First Officer had TERR ON ND selected on his side and I had the radar on for weather in the area. I had voiced concern over the low altitude we were assigned. Right after that we got a TERRAIN TERRAIN GPWS Warning to which I responded immediately by climbing at TOGA with full aft stick and verified spoilers stowed. We advised ATC and had climbed to 8000 Feet. After restoring automation; we resumed being vectored and descended slowly to get over the valley and gain more terrain separation than the minimum. Due to rapidly changing weather conditions and a very low altimeter of 29.26 inches; there may have been an altimeter error that caused the warning since we were at minimum vectoring altitude. In my opinion; vectoring off route should be done at higher than minimum with unusually low altimeter settings and in rapidly changing weather conditions.