You need to use different coordinates for your vertices since the coordinate systems are completely different. For a given vertical FOV and aspect ratio, you can calculate the range in X and Y direction at a certain depth Z.
yrange = Z*tan(fov/2); xrange = yrange*aspect;
The range of the coordinate system at depth Z is from -xrange to xrange in the X direction, and from -yrange to yrange in the Y direction.
But why do this and not use an orthographic projection for your 2D, and a perspective projection for your 3D?