It's absolutely possible to set separate credentials for a different VM even when using tags (or any container object for that matter), but it might not be clear how to do so. If, in the "Virtual Machines to backup" screen, you have selected a vSphere Tag, then when you go to the "Credentials" screen it will only display the tag, however, on this screen you can click "Add" and it will bring up the hierachy browse screen. On this screen you can select your tag again, expand it and it will show all the VMs with that tag. Select the specific VM (or VMs) that you need to override and click the "Add" button, and now the "Credentials" screen will have both your tag, and the individual VM listed, and you can override the credential for just that VM.
I hope that explanation is clear, I can try to do screenshots and upload if not.