Statement of Work (SOW) spend in many organizations dwarfs contract labor spend. The major VMS players lack the knowledge & capabilities to manage SOW spend effectively. They will tell you they do but when I talk to my customers who have tried to use them they tell me the VMS/MSP players are missing the mark.
From a technology perspective few can handle true SOW transactions. These transactions can be complex with detailed and many deliverables/milestones and acceptance criteria. Scope changes further exasperate the traditional VMS/MSP providers.
If you have SOW spend in your organization make sure you add it to your service provider decision criteria with at least 50% weight factor (or more).