its a bit confusing since it is according to the spec a unique identifier for the platform vendor. If it is the case it should be also accessible via clGetPlatformInfo(...).
Currently you have to get the device handle from the platform first to figure out what platform you are actually dealing with.
my test setup:
intel + amd sdk installed / AMD GPU
CPU CL_DEVICE_VENDOR_ID: 32902
CPU CL_DEVICE_VENDOR_ID: 4098 (which is the same CPU as above of course)
GPU CL_DEVICE_VENDOR_ID: 4098
am I missing something obvious?