There is only 1 VCA certification on Digital Business Transformation - the training is free, the exam is cheap, it's not a pre-requisite for any of the higher level certifications such as VCP - more on it here: VMware Certified Associate - Digital Business Transformation (VCA-DBT)
For VCP and above, there are 4 separate solution tracks - Data Center Virtualization (DCV), Desktop & Mobility (DTM), Network Virtualization (NV), and Cloud Management & Automation (CMA) - after you have your first VCP you can go deeper (with VCAP/VCDX) or broader (with multiple VCPs).
VCP certifications have 3 mandatory requirements:
1. Attend an authorised training course (the CBT Nuggets ones are not authorised)
2. Pass a vSphere Foundations exam (this is generic across all 4 solution tracks)
3. Pass the VCP exam
Have a good look through this mini-site and the various pages - it should answer most other questions for you: Certification
VCA-DCV Was retired as of Dec 31 2017 and the succeeding cert is VCA6-DCV, I had a lot of success and confidence after reading and quizzing myself with the Sybex brand, "VCA-DCV Study Guide book written by Robert Schmidt and Dane Charlton, couple that with the free online resource on the VMWare website:
Data Center Virtualization Fundamentals [V6]
Its a mock up exam of 25 questions simulated to the pool of questions you might closely receive on the actual exam, I would schedule to take the exam after I had received a 100% 5-6 times straight if I were you.
- GOOD LUCK