Yep, that bit [RedactedCo] hard two weeks ago; we kept wondering why vCenter would randomly stop responding on the web client, and we'd have to re-boot the vCenter machine. This all started after we upgraded the 6.0.[mumble] to 6.0U2 some months previous. It got to the point where none of the services would start, and I called in VMware on a 'OMGMAYDAY' priority ticket. (because not being to manage the environment actually *IS* an emergency around here...)
Turns out, 6.0U2 with an embedded PSC *and* a linked vCenter (also with an embedded PSC) is not a supported configuration. Your Humble poster got to build a new external PSC, A new vCenter, restore the vCenter database (after bodging the broken machine up far enough to get a database backup!) so that Distributed vSwitch would continue working, then re-connect hosts..
That was the main site; the second site that had the linked vCenter got a similar treatment (external PSC and new vCenter).
Thankfully, the only downtime we endured was having no automated HA or DRS shuffling while that was taking place, no granular backups of the environment, and our DR solution broke horribly as well, requiring a full reinstall. The business never noticed anything was wrong, and none of the applications they used skipped a beat.