PATH = VL_SETUP() adds the VLFeat Toolbox to MATLAB path and returns the path PATH to the VLFeat package.
VL_SETUP('USINGVL') adds to the path also the prefix-less version of each function (so, for instance, it is possible to use SIFT() instead of the more verbose VL_SIFT()).
VL_SETUP('QUIET') does not print the greeting message.