I have done a upgrade to 10.10 and Fusion 7
Everything has been fine, and I created a new custom vmnet, and some VMS - almost had everything working.
During the third VM that I made, I realised I needed to change the network so it was in the same vmnet.
Anyway, in the settings for the network its all greyed out - but all other settings for the VM are fine.
I figured it was a problem just for this VM, but no, all the VMs have the sam issue!
Also when I go to the Fusion preferences, and go to the network tab - all this is greyed out too. The other tabs are ok.
It seems as though just the networking element of fusion is broken - all VMs work fine - I just can change VM settings, or add new VMs due to the same issue.
Ive tried stopping and starting the services... no luck
the log did reflect that vmnet-bridge is already running.
I also wonder if it has anything to do with Cisco Any Connect client that I have - this has carried over from the update from 10.9 to 10.10 - and when I connected to the VPN there was an auto update - though this may not be related at all.
I believe this happens because one or more of the low level processes that manage networking for Fusion (the vmnet-* processes) gets stuck or Fusion is otherwise unable to communicate with them. When that happens, Fusion can be restarted, network adapters removed and added again, but it still won't help because from Fusion's perspective, it has no control over low level network settings through those processes, so the settings are shown as being unavailable for editing.
When you restart Fusion, it does try to start those processes again under its own control, but it notices they're already running and stops trying, and Fusion ends up in this state where network services are in fact running, but the settings to control them are unavailable.
As a symptom of this issue, you can check system.log in Console and you may notice messages like:
VMware Fusion 2235595: Error: Unable to start the bridge services. Error: /Applications/VMware Fusion.app/Contents/Library/vmnet-bridge is already running, process ID: 2852 
To fix it, either reboot, or alternatively this should work:
As I said you'll probably get the same effect by just rebooting, but sometimes that's not an option for various reasons.
Once you do that, network settings should work again.
Clean uninstall and install of Fusion should help you. If the networking files are locked one will face symptoms similar to yours as well.
Note: If you have created any custom networks you will have to recreate them ( which I believe is not a complex task using network editor )
Refer to below KB articles for manual uninstall and install of Fusion
I solved this problem on my Mac (El Capitan) by renaming the Preferences folder and then restarting my Mac
The presences folder is called "VMware Fusion" located in ~/Library/Preferences/
Quite frustrating but this worked for me.
Two years later still suffering the same problem. Obviously something's 'locked' the network settings. The hassle is how to unlock it...
Two hours later...
Finally the network settings are not disabled. Running the VM, the thing can finally connect to the intarwebs.
Now to continue with the *work* I need to get done tonight...
I'm having this problem with Fusion Pro 10.1.3 on macOS 10.14.
It has been some time since I tinkered with these network preferences. When I recently opened the preferences, things were normal (not greyed out). When I returned (after making no changes), things have gone sideways.
I've tried killing the vmnet processes and removing the Preferences folder, but no luck.
I haven't tried reinstalling, which sounds rather drastic.
VMware, what say you in regard to this problem?
Make sure your Mac OS Gatekeeper setting is allowing VMware Fusion under System Perferences->Security & Privacy - towards the bottom under 'Allow apps downloaded from:' choose 'allow' for the related Fusion warning message. After allowing this, I was able to make changes to my Fusion Pro 10.1.5 network settings.