The VCDX certification is well known these days, and because of that, there are many more resources online to help you. Your first stop should be the blueprint that can be found at https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/certification/vmw-vcdx6-dcv-blueprint.pdf. You'll also want to review the handbook that can be found at https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/certification/vmw-vcdx6-dcv-handbook.pdf.

Beyond the official documents linked previously, the boot IT Architect: Foundation in the Art of Infrastructure Design: A Practical Guide for IT Architects by VCDX-001, John Arrasjid, is a good book to read. You can find it on Amazon.com at https://www.amazon.com/Architect-Foundation-Infrastructure-Practical-Architects/dp/0996647708/ref=sr_1_2?ie=UTF8&qid=1543255581&sr=8-2&keywords=the+it+architect.

VMware also offers VCDX workshops, held monthly, that educate candidates on the VCDX process and helps to prepare them for the application and defense. The best resources, however, will be VMware community members who are going through the same experiences as the VCDX candidate. You should use Twitter and the VMware Technology Network forums to connect with like-minded technologists who share the same goal of becoming a VCDX and work with them to review your documentation and application and hold mock defenses. Many successful VCDX holders will say that mock defenses helped them to achieve VCDX status.