Update: Since first posting this, I've been informed that VUM is able to patch the ESXi hosts, whether you're running 5.1.0 or 5.1.0a versions of vCenter. I infer one of two things from this: I went after the updates too soon (before VMware had published the update for VUM to use), or my VUM install isn't getting the update info correctly. This change only affects the way you (can) go about updating the host; the vCenter server upgrade doesn't change.
Note: The offline update package for 5.1.0a is not for use with VUM; you'll have to either install from the ISO or use command-line methods to perform an upgrade of ESXi. The latter will be covered in this post.
Reminder: If you run vCenter in a VM, not on a physical host, use out-of-band console access that bypasses the vCenter Server! As soon as that vCenter service stops—which it must—your connectivity to the VM goes away. You can use the VIC if you connect directly to the ESXi host that's running the vCenter VM; that's the way I do mine. Windows Remote Console should only be used with the "/admin" switch, and even then, your mileage may vary. Any other remote access technique that mimics the physical console access is fine. Just don't use the VIC or Web Client remote access via the vCenter services that you're trying to upgrade. "Duh" might be your first response to this, but the first time you forget and connect out of habit, you'll hopefully remember this post and smile.
As with all upgrades, vCenter is first on the list, and in the new model introduced with 5.1, that starts with the SSO Service. That was recognized as an upgrade, and proceeded and succeeded without any additional user entry beyond the obligatory EULA acceptance.
Just to be sure, after SSO finished updating, I tried logging in using the "old style" client (VIC) from the shortcut on the vCenter desktop: no problem. Then I tried it with the Web Client: failure. On a hunch, I restarted the Web Client Service, but with no luck: "Provided credentials are not valid."
One more test: I'd used the "Use Windows session authentication" option in the Web Client. This time, I tried using the same domain credentials, but manually enter them instead of using pass-through: Pass.
That may be a bug; it may be a problem with unmatched versions of SSO and Web Client. Moving on with the rest of the upgrade...
The next step is to upgrade the Inventory Service service; like SSO, it can upgrade without specific user input. However, when the service is replaced with the newer version, vCenter Server (and other services dependent on vCenter Service) is stopped and not restarted. Manually restarting the services will allow you back at your system again, just in case you get interrupted while working and need to get back on before updating the vCenter Server service to the new version...
Like the previous services, vCenter Server recognizes that it's an upgrade and click, click, click to complete. Naturally, the services are stopped in order to replace them, but the installer does restart them when it's done. Upgrading the VIC is another click,click,click operation, as is the Web Client.
It did not, however, fix the pass-through authentication issue in the Web Client.
I spent a while in conversation with Chris Wahl and Doug Baer on Twitter, trying to get it straightened out. Both are VCDX and super sharp, and they gave me lots of solid advice for improving bits of my vCenter setup, but this one wasn't budging. At this point, I've given up on it: there's a solid workaround, so it's not a dealbreaker. Watch this space, however: if/when I figure it out, I'll pass along my findings.VUM isn't updated in this release, so that bit doesn't need to be upgraded or reinstalled. However, the offline package isn't going to work with it (as mentioned above), so the upgrade is done using one of the alternate methods. My preferred choice is to use the busybox shell via SSH.
To use this method, I first used the VIC to upload the offline update to a datastore visible to all my hosts. Next, I put the first host into Maintenance Mode. Because I took advantage of the sweet "ssh AutoConnect" plugin, the console of the host is a right-click away. Once at the console, the following command is executed:
esxcli software vib update -d /vmfs/volumes/[DATASTORE]/[PATCH_FILE].zip
After a short wait, the prompt was back informing me that the update was complete, and a reboot was required to implement. You can use your favorite method of restarting the host, and once it returns to connectivity with vCenter, you have to manually exit Maintenance Mode. Repeat on each host until you're fully updated.
This update didn't replace Tools with a new version; the tools installed as part of the GA version are recognized as current, so I didn't get to see if the promise of no-reboot Tools updates would come to fruition.