Possible workaround:

After editing the VM's .vmx file to add

mouse.vusb.enable = "TRUE"

mouse.vusb.useBasicMouse = "FALSE"

usb.generic.allowHID = "TRUE"

you'll be able to use your five-button mouse in the VM. To do so, open your VM, then navigate to Virtual Machine > Removable Devices > [Your Mouse] > Connect (Disconnect from Host). You'll get a couple warnings that you won't be able to use your mouse with the host anymore, and just click OK.

Now you can only use your mouse within the VM whether it's full-screen or in windowed mode. Hit Ctrl-G on your keyboard to get rid of the host mouse icon and fully immerse yourself in the VM. Once you need your mouse again outside the VM, hit Ctrl+Alt, then release. Now the focus is back to your host OS. Hold Alt and push V to bring up the Virtual Machine menu, and use your arrow keys to navigate to Virtual Machine > Removable Devices > [Your Mouse] > Disconnect (Connect to Host).

Now everything is back to normal. Hopefully something will change in either Ubuntu or VMWare Player soon to make it easier than this.