VMware has recently released vCloud Availability 220.127.116.11 update that adds vCloud Director 18.104.22.168 compatibility. The tenant needs to install vSphere Replication 8.1.1 which supports vSphere 6.7U1 all the way down to 6.0U3.
The on-prem upgrade from older vSphere Replication appliance (e.g. 6.5.1) is side-by-side. Meaning; you deploy the new 8.1.1 appliance and it connects to the existing one to migrate the data over.
I have noticed that with the new 8.1.1 appliance my cloud replications were not active.
The reason for that was the vcta service was not running on the appliance. The service is responsible for establishing the tunnel with the cloud endpoint and transferring replicated data. Note that the service is not needed for regular vSphere to vSphere replications.
In lab environment where you need to apply a custom endpoint certificate as described here, you might not notice this issue immediately, as the service is started after the certificate change manually with service vcta restart. However, after appliance reboot the service will be down again.
The fix is easy, just enable the service with:
systemctl enable vcta
command from the appliance CLI (via console or SSH if you enabled it before). This is one more thing to remember when setting up cloud replications next to the ESXi vr2c-firewall.vib issue I documented here.