On Sandy Bridge CPUs (as well as on later chips, empirically), you can read the "Core Voltage" from bits 47:32 of the MSR_PERF_STATUS MSR (0x198, seems to be the same as IA32_PERF_STATUS, not what the meaning of the different prefixes "MSR" vs "IA32" mean).
SDM volume 4 describes this as (note the typo 37:32, it should be 47:32):
Core Voltage (R/O)
P-state core voltage can be computed by
MSR_PERF_STATUS[37:32] * (float) 1/(2^13).
What isn't clear if this is the VID (voltage requested from the voltage regulator by the CPU), or the measured actual voltage "vcore" or "vcc" at the core. Normally the two will be close, but the not the same.
Various tools refer to it as the VID, while others call it "core voltage". The SDM text would make one lean slightly towards VID as it says "P-state core voltage", where the reference to p-state makes one thing it is the required voltage associated with a p-state (i.e., the VID) rather than a measured, actual voltage.
Note that bits 7:0 of this MSR used to return the VID, but not on any more (on Skylake, they seem to always be zero). Bits 15:8 do seem to return the FID (frequency ID, i.e., an ID which maps directly to the frequency multiplier).