That alert you have there is a Fault type, driven by the Guest OS | Free Space(GB). This attribute group is populated by the VM Tools and reported to vSphere/vCenter. If you want to catch it BEFORE it gets 0-bytes free, you'll want to setup a hard threshold on the attribute package applied to your VMs, using either a hard threshold GB free or a %.. % free/used being easier to apply to a large set of VMs and keeping things relative. This is assuming you have vC Ops Advanced+, as you need to access the Custom UI to edit these packages and set up these HTs and associated alarms.
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