Hey John,
I'm not too familiar with the vCenter Appliance, I only use it for testing normally, but if you're using the embedded database it's only configured for up to 5 hosts and 50 VMs. Since "not big" is relative, it's possible the issue is just size of the environment.
In any case, VPXD is the vCenter Service itself, so it's literally vCenter creating that high load. As to why it's creating the high load, my first guess is taking a look at the DB and whether it's on that appliance. I'd also try restarting the VPXD service if you haven't in case it's hung on something.
~Brandit