VTune Performance Analyzer uses the Sampling collector driver to collect PMU events in the Core of a microprocessor. Very simplified description is: microprocessors PMU is programmed by VTune to count an event(s), and when the counter exceed a threshold [which is set by user], PMU generates HW interruption which in its turn caught by the drivers interruption handler and gathered IP, PID, TIP and some other context information this we call Sampling. This works for all CORE events, meaning all the counters that are located in the CPU cores.
There are UNCORE events, which are being counted outside any CPU core, e.g. memory agents, etc. In fact, there is no sense in collecting CPU core context information when Un-core counter exceeds some threshold we couldnt map that event to any of the Cores for the moment, so the context information would be useless. Therefore, un-core events are not supported in Sampling mode.
However, un-core events can be collected in counting mode. Its not trivial and requires some additional efforts. Please, refer to the Intel 64 and IA-32 Architectures Software Developers Manual (Uncore Performance Monitoring Management Facility chapter).
See the example of measuring memory bandwidth on Nehalem processor:http://software.intel.com/en-us/articles/how-do-i-measure-memory-bandwidth-on-an-intel-core-i7-or-xeon-5500-series-platform-using-intel-vtune-performance-analyzer/