Update: VMware has released a patch for vCenter which fixes the issue – release notes.
Recently we noticed that logs from vCenter did not reach our log servers. When I restarted rsyslog on the vCenter appliance they started to work again but after some time it stopped again. There is a thread in VMware community about this issue – https://communities.vmware.com/thread/618178. In short the fix most likely will be available in next patch for vCenter. There is also described in the thread a workaround how to manually update rsyslog package on the appliance.