The Design Defense phase is 75 minutes. You should be prepared with a presentation highlighting the business use-case, key requirements/constraints/risks, high-level design, and logical design details for the critical portions of the blueprint.
The Design Scenario phase is 45 minutes. You will be given a hypothetical business use-case and you are expected to derive the key requirements and work through a high-level design using the physical whiteboard or digital whiteboard if performing a remote defense.
There is no correct answer here. The design documentation needs to include every area covered in the VCDX Blueprint for the given track you are applying for. The documentation should be sure to provide enough detail to allow another architect or VMware Administrator/SRE to be able to understand the key areas of the environment and be able to reason out why each design decision was made.
If you were to only pick one additional resource beyond the VCDX Blueprint, I recommend to everyone to purchase and read the IT Architect: Foundation in the Art of Infrastructure Design book (available on Amazon). The book covers all of the key areas in the blueprints, makes recommendations on design documentation structure, how to create the presentation, etc.
The design decisions are the core of the VCDX certification. A candidate needs to be able to show mastery of the process and be able to justify their individual decisions. At a minimum, I recommend including the justification, impact, identified risks, and risk mitigation for each design decision.
The design documentation should include diagrams for the conceptual, logical, and physical portions of the design. At a minimum, these should be included for the compute, networking, and storage sections.
The presentation will be the talk track for the 75 minute design defense portion with the VCDX Panelists. The presentation should highlight the key business requirements, conceptual, logical and physical portions of the design. It can also include reference slides for other critical aspects of the design to assist in answering the panelists questions.
The VCDX community is very welcoming. The main resource for finding a mentor should be the vcdx.vmware.com website. In addition, the hashtag #VCDX on Twitter is also a good way to reach out and find individuals willing to mentor you during your process.