Humans in the Digital Age are continuously exploring different forms of socializing on-line. Social 3D Virtual Worlds provide an alternative that are gaining in popularity. They constitute virtual environments where people freely socialize by participating in open-ended activities. Moreover, Virtual Worlds can also be used to engage humans in e-* (e-government, e-learning, e-commerce) applications, the so called Serious Virtual Worlds. Implicitly, these serious applications have specific goals that require structured environments where participants play specific roles and perform activities by following well-defined protocols and norms. In this paper we advocate for the use of Virtual Institutions to provide explicit structure to current Social 3D Virtual Worlds. We refer to the resulting system as hybrid (participants can be both human and software agents) and structured Virtual Environment. Specifically, we present v-mWater (a water market, an e-government application deployed as a Virtual Institution), the infrastructure that supports participants' interactions, and the evaluation of its usability.