The vsv's you are asking about are generally in the forward portion of the compressor section in the engine. The vanes rotate (twist) to optimize the efficiency of the compressor across the operating conditions of the engine, not just the cruise condition. By being able to rotate (open or close) the stationary vanes, the total performance of the compressor can be optimized while maintaining acceptable stall margins.
You may have had a typo. The vsv's control the angle of attack of airflow directed to stationary vanes (every other vane is variable attacking the stationary). They are designed to allow for the maximum amount of compression W/O stalling the airflow over fixed vanes. VBV's bleed the compression for the same effect. The VSV's are generally located in the compressor only and are every other blade to the fixed compressor vane.