You'll find the various paths of requirements (as they depend on your current certifications) on the pages for each certification:
Looks to me like you can achieve VCP-DCV 2019 just by passing one exam, but to achieve VCP-CMA 2019 would require you to a different VCP first.
You don't need an exam voucher, you can pay for exams directly online using a credit card.
VCA-DCV doesn't exist any more, the only VCA is the DBT, the exam is not free though: VMware Certified Associate - Digital Business Transformation (VCA-DBT)
Since the recertification policy has been removed in again, you may just need to pass the exam for one or the other certification.
I'd suggest you explore the options at https://www.vmware.com/education-services/certification.html to find out what you need to do to achieve your goals.
This site also contains a link on how to purchase vouchers.
I have not attended VCP-DCV 6.7 Class room training but i have attended VCP-CMA 7.3 class room training so can I book my VCP-DCV 6.7 and VCP-CMA exam directly ? or any other prerequisites are there?
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You can answer those questions from the links I posted previously - select the path on each page that applies to your current situation.
Your existing VCP means you can achieve VCP-DCV 2019 by passing just one exam (as per the "VCP5 or VCP6-DCV path")
To achieve VCP-CMA 2019, you would need to pass a vSphere Foundations exam as well as passing the VCP-CMA 2019 exam (as per the "no VCP certifications" path, you'll notice there is no "VCP5-DCV" path option)