Metaphysical/ontological vagueness, classically understood, can be summed up as the problem of borderline cases of some thing x. Clouds are the standard examples of vague objects. If posit that our minds meet up with some things in the world that we call "clouds" that have something akin to definite boundaries beyond which an existential quantifier is rendered false, then what are we to say about the quantities of material that constitute clouds (i.e. water droplets) that fall outside the boundaries? Some (e.g. Williamson) argue that there is something like an epistemic cap to the question: clouds do exist and there is a real boundary between a cloud and not-a cloud, but we will never be able to find it because it is simply beyond our perceptual or reasoning capacities. Others argue for what is called "supervaluationism" or super truth – the idea that truth values are not characterized by the "T or F" dichotomy, but that there is some metaphysical commitment of philosophers to some sort of truth gradient. Still others (e.g. Morreau) argue that there are real vague objects.
Let's work from the "there are real vague objects" camp.
Vagueness is generally seen as an problem wherein predicates can be said to satisfy their descriptions. Borderline cases are observable for all of the following: "George is bald", "Ship A is not the same ship as ship B", and "The table is made of wood". Great. Note though that certain descriptions which sit in the functional "place" of the predicate are not themselves predicates, such as "exists". This is observed by Kant, among others. What this means, for our purposes here, is that existence is not a predicate. What this means, in turn, is that there is something about the logical operation of existential quantification that is somehow more "basic" or "atomic" or metaphysically "true" about positing the existence of something, such that that quantification does not count as a description. Now, let us look at the sorites paradox, a.k.a the paradox of the heap. Take a grain of sand. Is this a mound? Probably, the answer is no. Add another grain of sand. Is this combination a heap? Again, the answer is no. From this, it can be logically deduced (via modus ponens and the logical operation "cut") that adding a single grain of sand will never make the difference between a given non-heap collection of sand and a real heap. The conclusion to be drawn then is that there are no such x's that are heaps of sand. This is patently false. If you don't believe me, then pour a cup of sand onto your desk and tell me that there is no heap on your desk. Thus the paradox.
One way to get around this would to say that it is only a heap because we say it is. But the question I wish to ask is then how, if the material the constitutes the heap exists, and existence is not a predicate, can that material only be a heap when we say it is? If this is true, which is to say, if there is only a heap when we say there is a heap, then it seems that there must be borderline cases of existence, i.e., collections of sand that are possible but not actual candidates for extant heaps of sand, no? But if there are such cases, then existence is a predicate, and there would be no reason to posit the question. However, what are the metaphysical and ontological ramifications, if existence really is a predicate? Is existence to be understood as something like "red" or "bald", which is to say, only true as specifiable by human beings? Is this a deflationary take on truth? I don't know where to go next.