Thank you for your response. esxtop shows no swapping or compression on the host which is running the vcenter VM.
VMware support has pointed me towards VDP possibly being at fault (what else is new?). Based on this kb it looks like I will need to upgrade to update 2 to allow us to upgrade VDP to 6.0.
They are still investigating our high logging situation as well. I will update if and when they find anything else.