PCMark Vantage is based on the hard drive usage of actual applications, albeit applications from the Windows Vista era. Futuremark uses so-called traces, which are recordings of disk activity of real programmes, and repeats the same activity as quickly as possible in the benchmark.
A score of about 42,000 is simply disappointing, and also unexpected, considering the Vertex 4's excellent results in other benchmarks. OCZ's engineers provided this explanation:
The reason why you are seeing a lower performance has a couple of reasons; PC Mark (Vantage, Ed.) is mostly based on compressed files performance which means it’s not a good representation of the capabilities of the Vertex 4. PC Mark also intentionally keeps a low queue depth in order to simulate real world performance. However PC Mark also specifically runs unaligned I/O which is not simulating user experience as a majority of the I/O in Windows 7 is aligned.