This is expected behavior when adding a brand new host that vCenter hasn't connected with before. vCenter is double-checking the authenticity of the host you're trying to connect to before adding the SHA1 key to its known-hosts.
Regardless, the logs you're looking for are /var/log/vmware/vpx/vpxa.log and /var/log/vmware/hostd.log
The path /var/log/vmware exists.
But, vpx folder and hostd.log does not exist in my system. vpxd folder exists.
Sorry I was searching in vcenter, both logs exist in the ESXi.