Doing 2D in OpenGL is pretty simple, and just using glVertex3f doesn't automatically mean it's 3D.

- Switch to orthogonal projection and use your screen size as the parameters (or a set virtual screen size if you want it to look the same everywhere)
- Call glVertex3f with (virtual) screen coordinates like you'd draw in 2D. Use the Z-Coordinate to get free sorting

Example :

Code:
glOrtho(0, 1280, 720, 0, -16, 16);
glBegin(GL_QUADS);
 glVertex3f(100, 100, 0);
 glVertex3f(100, 300, 0);
 glVertex3f(300, 300, 0);
 glVertex3f(300, 100, 0);
glEnd;
// Draw another quad ontop. OpenGL will automatically sort if depth test is enabled
glBegin(GL_QUADS);
 glVertex3f(200, 200, 1);
 glVertex3f(200, 500, 1);
 glVertex3f(500, 500, 1);
 glVertex3f(500, 200, 1);
glEnd;