For me, the most implausible kind of non-epistemic vagueness is vagueness about the existence of substances: on the kind of hylomorphic view of substances that I like, it is very difficult to make sense of the claim that there can be any non-epistemic vagueness as to how many substances there are, for instance.
But there are reasons for thinking that there is vagueness as to the existence of substances. George is a deer and deer are substances. George is dying. Is it very plausible that for every time t during the process of dying there is a fact whether George is already dead or not? After all, George's bodily state at t and t+e for a very small e will be extremely similar.
I find interesting here that the B-theorist can say something helpful here that the A-theorist cannot. Vagueness as to when exactly George perishes need not be taken to be vagueness about existence for the B-theorist: George exists in the primary sense of "exists", namely the tenseless sense. Rather, it is vagueness about which parts of space-time are occupied by George. (One can say this whether one is an endurantist or a perdurantist.) But of course for the A-theorist, to be non-existent now is to be non-existent. There are some complications here in that some A-theorists are committed to the existence of past and future substances. But I am convinced by considerations similar to those in Zimmerman's piece on the A- and B-theories that the A-theorist should affirm truth simpliciter to the proposition that, say, George is now non-existent, where the "is" is present-tensed, while the B-theorist will make this be some kind of relative or derivative truth.
One might think that vagueness as to when a substances comes to be can generate vagueness about existence even for a B-theorist. Plausibly, there is vagueness as to when George came into existence (presumably, it was when the gametes united, but the precise point at which the gametes count as having united may be vague). But then we can ask which time t is such that were all biological material in the universe destroyed at t, then it would be the case that George never existed. And there seems to be vagueness about that.
But perhaps vagueness about that can be vagueness about counterfactuals rather than vagueness about existence?