In the past I've seen virtual machines where their network adapter appeared to be connected under Edit Settings in vCenter but the OS was not registering any network adapter being plugged in.
In particular this issue would happen after a system reset. The fix was to hop into vCenter > VM > Edit Settings > and disconnect the network (uncheck the connected box), hit ok, then go back to edit settings and check the connected box next to the NIC. This would always fix the problem.
Fast forward to this environment I'm in today, I am seeing the same thing happening after Instant Clones are being provisioned off of the snapshot. The only way to fix it is to log into vCenter and toggle the NIC as described above.
I havn't had a chance to update vCenter, ESXi, the vDistributed Switches, and the VM Hardware all of which is on my eventual bucket list.
Have you seen this before?
Check the physical switch port configuration. This is a common issue if e.g. "spanning-tree portfast" is not configured. If this is not configured the spanning-tree algorithm can take up to 45 sec. which causes the virtual network connection to time out.