Submitted VCDX designs may be fictitious, either partially or wholly. But VCDX candidates must defend all parts of their designs as if they were real. VCDX panelists do not ease their questioning on fictitious parts: if they did, candidates could evade questioning on weaker parts of their designs by claiming, truly or falsely, that they were fictitious.
I'd like to echo and support Brian's comments here. I've had discussions on this topic with several people considering the VCDX defense. I find it much easier to defend a real or mostly-real design due to the simple fact that you probably had to go through many of the discussion/questiion points along the way. For a fictional design, the work usually springs from your head fully formed, lacking the the realness that makes an implemented design defensible. Or, as Brian so eloquently states, "Sustaining a complex, internally consistent fiction is more cognitively difficult than describing a set of facts."
If you plan to defend a made-up design, remember that "because that's the best practice" is a terrible answer to any question regarding why you designed something a certain way. You must understand each decision point and why you chose a certain path over the other(s). In my opinion, that is much easier when the design was created with a customer who has to own the solution once it is implemented. They tend to understand what is required to have something supportable.
As Brian mentions, and a lot of candidates fail to realize, the VCDX is as much about business requirements as technical solutions. In my opinion, the the defense leans more towards the business than the technical. Sure, a VCDX must understand the technologies, but the why behind the technology is arguably more important. If you've made it to the defense, you've already shown your technical understanding via the DCA and DCD exams. Now is the time to show that you can apply that to solving business challenges and present that solution in an effective manner.
Definitely run your design by co-workers -- they will probably ask some of the questions that the panel will ask. When the design is in your head, you think you've covered everything, and believe you have all the answers. Unless you are a truly unique individual, you have probably missed something. It is better to be prepared than to get caught off guard. Odds are you'll be nervous, and being prepared is the best way to handle that, IMO.
Brian, fantastic suggestions and great guidance!
Brian, you are churning out a nice series of helpful tips for candidates. Nicely done.