I am currently in the same path as what you decribed.
I am doing a rollback on the nic driver to ESXi 5.0 version.
Uploaded the zip file to my datastore and running it from the console.
It seems to work as I am able to get an IP provided by the router and able to ping outside.
Although I am doing all this on a Lab setup, I will move to the actual server when I see positive results.
So far, everything looks good. YOur latest response has more information on what I have gathered previously.
I will try to upgrade the host and later roll back the NIC drivers by "esxcli software vib install"
I believe the root of this is that the NIC driver on ESXi 5.1 has changed the architecture providing more features.
I will read more about your post and follow your advise.
Thank you for this post.