According to https://kb.vmware.com/s/article/2108294 for VMware vCenter you can download all certificate files including root ca file as followed:
Note: Direct URL to download the Certificate is https://vCenter_FQDN/certs/download.zip (Example - https://vcenter.domain.com/certs/download.zip).
How can I download / export all certificate on a NSX-T manager node?
So under the hood of the nsx manager appliance it's all ubuntu, with the ca-certificates package installed. Not saying this is what you are after, but I haven't been able to find a nice way of doing this yet.
On the appliance as root, you could run this - openssl s_client -showcerts -verify 5 -connect nsxmgr:443 < /dev/null, the certs should be at the top of the list. I think they may be the ones you need. You could save them to a file.
Or GET https://nsxmgr/v1/trust-management/certificates/ may have the one you are after.
Again not too sure if this is what you are after, just something I have found.