Have a look at this page: VMware Certification Roadmap for 2015
With vSphere 5, you can achieve a VCAP with a single exam, with vSphere 6 you will need to pass 2 exams to gain a VCIX.
There will be migration paths for those who have VCP and VCAP in v5, you can read more about that too: Migration Paths from v5 Certification to v6 | VMware Education and Certification Blog - VMware Blogs
So, you could wait and go straight for VCIX6. Or you could go for a VCAP5 now, and end up at VCIX6.
Either way, as long as your VCP is current (ie. it has not expired) you won't need to take any training.
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VMware Training & Certification blog
