There's nothing to stop you taking ("writing") the exams - anyone can take any VCP level exam - it's whether you would have met all the requirements to be awarded the certification or not that is key.
I'm answering your questions below as if you had asked if you would be certified for VCP6-NV:
If my CCNA is expired, can I write VCP6-NV?
No - VMware will validate your certification, as mentioned here: How to Request Verification of your Cisco Certification
If i have VCA-DBT, can i write VCP6-NV?
No - the VCA certification is not connected to any of the VCPs.
You can review the various paths for VCP6-NV here: VMware Certified Professional 6 – Network Virtualization (VCP6-NV)
Based on your questions, your path would be "no VCP certifications" - meaning you'd have to attend training plus pass 2 exams.