The vSphere content pack comes with setup instructions -- from the /contentpack page, select vSphere, select the gear icon, and select Setup Instructions. You will see for VC you must use the LI agent with the included agent groups. Syslog will not work with the content pack. I hope this helps.
vCenter Server Application Logs
The Log Insight agent should be installed on vCenter Server -- either Windows or Virtual Appliance. For Virtual Appliance instances running vSphere 6.x the vSphere 6.x - VCSA (Linux) Essential agent group should be enabled and configured. The vSphere 6.x - VCSA (Linux) Complete agent group can be used for more comprehensive vCenter Server application log collection though the vSphere content pack does not leverage these additional log files today.
Thanks Steve. Appreciate you taking the time to reply. I was able to:-
Install the Linux agent on my PSC and vCenter
Enable the vSphere 6.x - VCSA (Linux) Essential agent group and filter on OS.
I can now see the VPXd logs and a lot more too!