Vertex editing

Hey all, I''ve been trying to implement vertex editing in my program and I have run into a little problem. What I normally do is transform all my vertices with a VertexBuffer into screen space. Then I am able to detect which vertex was selected. Now, in order to edit the vertex, I transform the mouse point with the inverse of the world, view, projection and viewport matrices. With this, I get the model coordinates. The problem is, my "depth" value is always off..Basically, is there another method to edit vertices than the one I outlined above? Georges