I have one local user for the backup server to access vCenter and is there anyway to disable password expiration for only that user?
Assuming you are using a SSO user account then you can SSH into your VCSA and execute the following. Note you will need your SSO Administrator credentials:
me@vcsap01 [ ~ ]# cd /usr/lib/vmware-vmafd-bin
me@vcsap01 [ /usr/lib/vmware-vmafd/bin ]# ./dir-cli user modify --account test --password-never-expires
Enter password for administrator@[your SSO domain]:
Password set to never expire for [test].
Hope this helps.