my sapphire xtreme 5830 does not have a VRM trmp sensor (that I know of anyway, and GPUZ does not report anything that looks like a VRM temp).
In GPU-z its called VCC Phase I think. But not all cards support it. If yours doesnt, that would have me worried:
I use an aftermarket cooler on my 5850. I use the standard base plate to keep the ram and VRMs cool. With the stock cooler, even at 100%, I had much higher GPU temps and comparable (but still slightly higher) VRM temps than what I have now.
At stock voltage and stock speed my VRMs go to ~75C, while my GPU itself is no hotter than 45C. Overclocked to 900 MHz (from 725) and still at stock voltage, VRMs are closer to 90C, GPU temp is still only 50C. 90C seems too high for me, even if they are supposedly rated for 120C, so I undervolt and clock my card at 850.
THose of you overclocking and particularly overvolting should really worry about those VRMs I think. Its not difficult to keep the GPU cool, but I wouldnt be surprised if you were hitting way above 100C on the VRMs. I read somewhere on this forum you can buy IR thermometer for a $10, Id consider doing just that if your card doesnt support VRM monitoring.