How to get VMware Fusion on Mac to use Logitech extra back/forward mouse buttons in Windows VM?
To expand on what CajunLuke said about changing the configuration on a per app basis.

If you have Logitech Control Center for OS X installed you can use Advanced click settings for just VMFusion using the following steps.


System Settings > Control Center





Click the Plus button next to the global settings drop down and navigate to VMFusion and click select
Change Thumb Back button to Advanced Click with the button 4 option and Change Tumb Forward button to Advanced Click with the button 5 option.




I use win + left and win + right for screen snapping in windows, so key mapping was not an option for me.  Advanced click allowed me to keep the windows shortcuts intact as well as use my forward and back buttons on my Anywhere Mouse MX as intended. I have tested this in Win8 full screen and single window and it works beautifully.  I'm not sure about unity because I haven't used Win8 in unity mode so I did not test.  Hope this helps others.  I know it took me forever to find the anser I needed.