With the rise of cloud computing, enterprises increasingly rely for their daily operations on heterogeneous externally-sourced cloud services that span different levels of capability. Their IT environment is thus progressively transformed into an ecosystem of intertwined infrastructure, platform, and application services. To effectively manage the ensuing complexity, enterprises are anticipated to increasingly rely on cloud service brokerage (CSB). This work presents a conceptual architecture for a framework which provides solutions with respect to the quality assurance and optimisation dimensions of CSB in the context of virtual enterprises. The framework revolves around three general themes, namely governance and quality control, failure prevention and recovery, and optimisation.