Recently an Cloud Infrastructure Architecture Case Study has been released. This case study takes a VCDX type of approach listing design considerations, constraints, requirements etc. I believe that this document is a good indication of what a design document should look like and how it should be structured. I would like to point out that this case study should be considered as a guidance and not as a source of truth or be seen as a full VCDX architectural document. This document does not go to the level of depth expected, in my opinion, but does give a great idea of what would be expected.
http://www.vmware.com/resources/techresources/10255
The VMware Cloud Infrastructure Architecture Case Study Series was developed to provide an understanding of the various components of the VMware Cloud Infrastructure Suite. The goal is to explain how these components can be used in specific scenarios, which are based on real-world customer examples and therefore contain real-world requirements and constraints. The VMware Cloud Infrastructure Suite consists of five technologies that together expand the capabilities and value that customers can realize from a virtualized infrastructure. This case study focuses on vSphere 5.0 and vShield App 5.0