I've just tested a similar config as what you described. It works fine for me.
- My OS X power settings have "Automatic Graphics Switching" unchecked. This forces the Macbook to use high-performance graphics.
- In OSX Display settings, the Arrangement of my displays shows the dock/menu on the External display, not the macbook display.
- In Fusion, the VM settings for Display have "Accelerate 3D Graphics" enabled/On.
Not sure if those options matter, but with those options set, it works.
Also other trouble shooting:
It may be as simple as reinstalling VMware tools....
Troubleshooting Unity and User Access Control issues in Fusion
Unity mode is incompatible with UAC prompts. If you experience this issue, you may want to disable UAC.
After UAC is turned off and your virtual machine restarts, Unity works properly.