What version of ESXi are you using?
I am personally using 6.5.
I did not try and add interfaces after the fact, but rather provisioned the VM with multiple interfaces from the get go.
If you happen to be using 5, this is not supported with R80.10 per: Compatible Hardware List -- Virtual Machines (though 5.5 is supported)
Just as a test, I took a R80.10 VM I had set up and added an interface to it with the VM powered down.
When I booted it up, the second interface was recognized.
Make sure the Guest OS is set to RHEL 5 (64-bit) and compatibility is set to the latest hardware version supported.