Sooo can I use the pass for the 6.5 foundations exam as entry for the new VCP-DCV19 exam!?
vSphere Foundations on 6.0, 6.5, and 6.7 all count towards VCP-DCV 2019, confirmed here: VMware Certified Professional - Data Center Virtualization 2019 (VCP-DCV 2019)
Also will my exam voucher for the VCP6.5-DCV be usable for the VCP-DCV 2019 exam as it's doesn't expire until 2020?
As the cost of the VCP exams is the same, I expect your voucher would be OK. If you got your voucher from a college as part of completing their training (rather than a commercial training company), just be sure to do the "PSE" version of the VCP exam. Email firstname.lastname@example.org if you want clarification.