I'd argue that you're conflating philosophy and religion, and that unlike what you're suggesting by that conflation, much of philosophy isn't vague - it's careful and well thought out. Here's a random passage from Decarte's Meditations:

I have always considered that the two questions respecting God and the Soul were the chief of those that ought to be demonstrated by philosophical rather than theological argument. For although it is quite enough for us faithful ones to accept by means of faith the fact that the human soul does not perish with the body, and that God exists, it certainly does not seem possible ever to persuade infidels of any religion, indeed, we may almost say, of any moral virtue, unless, to begin with, we prove these two facts by means of the natural reason.

What about this is vague? He's clearly laying out his goals and how he intends to do it. If you want to say that religion, as a particulary subset of philosophical thought, is often vague, then sure, but a lot of philosophy is very intentional and focused. It's setting out a goal and working towards achieving said goal, not trying to write in a manner that will appeal to everyone.