Hi,
There is no iso file for macOS Sierra that you can just get. Instead the easiest way is to download the installer via the mac app store.
This installer then sits in your Applications folder.
You can drag and drop the installer on VMware Fusion instead of using an iso file.
Steps:
In the menu, select File -> New -> Keep the default "Install from disc or image", click the Continue button.
You get a screen with the text "Choose an installation disc or image" and that's where you can drag and drop the installer.
--
Wil