VDC refers to Virtual Design and Construction. It describes the ideal process in which a building is built twice: the first time digitally using a BIM model, and the second time, physically as in the actual construction. Constructing a building is a complex process in which there are many tasks and activities, and the key idea is that all these tasks and activities are simulated digitally before the actual physical construction to cut down on the time, cost and resources due to mistakes. This concept hinges on the assumption that making a mistake in the digital world is cheaper than making a mistake in the physical reality.
IDD refers to Integrated Digital Delivery. It refers to the process in which a digital model is the main mode of communication throughout the lifecycle of the building, starting from the planning requirements of a new project, and throughout the facilities management of the building, until the demolition phase (end of lifecycle). Using a BIM model throughout the lifecycle of a building makes the initial investment in creating the BIM model more worthwhile, because the maintenance of the building goes on for a far longer period than the construction phase of the project.
Tying it all together: VDC and IDD are both processes which describe the different ways in which a BIM model is used throughout a construction project and the lifecycle of the building. Both processes require that a BIM model contains accurate and up-to-date information at each stage of the project or building lifecycle. BIM is database of information related to a building, and it forms the digital backbone of the VDC and IDD processes. In order to execute VDC and IDD productively, care must be taken to ensure that the BIM model is consistently updated with the latest information, and that the BIM team has implemented quality check workflows.