ProductXport component export Google taxonomy as string. If you need to export it as id, you can have is stored this ID manually in the VirtueMart as custom field. Let us say you have it stored somewhere here:
See the number of the field - it's important for next setting.
Open the component productXport - setting for VirtueMart, open the tab "Advanced options" and enable "Export custom fields". Then find the custom field you wish to export and enable this one.
Go to the server setting, open one of the export (ie google.com) and add new user defined field.
The last step is to setup this field to the VirtueMart custom field like this: