I think you're absolutely right about fortran's legacy advantage (that, and it's really not a terrible language for numerical work) but the distinction between 'supercomputer' and cluster of commodity PC's is all but history. Virtually all current supercomputers are clusters made up of the exact same chips you can get in desktop machines. The vector processing optimizations you mention are merely SSE or AltiVec or a GPU, technologies we all have access to. The only thing that makes a supercomputer super is the number of processors and the interconnect.