Thanks Mohammed. I've tried connecting to both vCenters by IP as well as DNS name without success. I also verified that the account I used could connect to vCenter through the fat client. I also ssh'd into the appliance and verified that I could do DNS resolutions and could ping both vCenters - they're on the same subnet.
I've got the log files but obviously they're huge. I haven't found anything in there that suggests a plugin registration failure. Which string should I search for?