@boagie,
Any given concept within the category of theism may be either clear or vague, depending on the individual doing the conceptualizing. Clarity of a concept, however, does not necessarily mean the concept is definable in concrete terms. Humanity, for example, is a clear concept. It is also a concept that defies definition in concrete terms unless you want to reduce humanity to a race of one of the many species of automatons on the face of the earth (which, of course, is one approach that one may choose). Humanity is ultimately a mystery. So is the earth. So is the Universe.
So, if I use the term GOD to indicate, symbolize, or represent in some way "the ultimate creative force behind the universe," or "ultimate reality" or "the ground of all being" or some other attempt to convey what is an ultimate mystery, I am not necessarliy either hiding behind vagueness, nor am I necessarily pretending that I know exactly what I am talking about--the two errors that theists are alternately accused of.
It seems worth noticing too, that when a conception of theism leans toward definitiveness, it is subject to the one charge, and when it leans in the other direction, it is subject to the other. Some, it seems, would set the rules of the debate so that there is no possible way for theism to win.
In the final analysis, though, what seems most important is that theists are pointing to
something that is a reality--whether or not you choose to call it "GOD." The various conceptualizations and models of the various theists lie along a continuum that ranges from immature to profound, but for all of them, it is a reality, in that it is something they experience and try to interpret.