That doesn't sound like a very flexible solution. Normally you would disable Z-testing, and then just draw the individual components in the order as you mentioned. It's possible to put everything in one (or more) VBO's, and render subparts though. By using offsets, or indices. For example, if you go for indices, you could:
- Create 1 VBO containing ALL geometry
- For each thing, create a buffer with the indices (usually 4 ints referring to the corner vertices)
- Activate the VBO
- apply border material, render border , using the indices that refer to the border quad


glBindBufferARB( GL_ELEMENT_ARRAY_BUFFER_ARB, myIndicesBufferObject ); // bind indices, also stored on the GPU
glDrawElements( GL_TRIANGLES, count, GL_UNSIGNED_SHORT, 0 ); // render
glBindBufferARB( GL_ELEMENT_ARRAY_BUFFER_ARB, 0);

- apply background material, render background, using the indices that refer to the background quad
- ...and so on...
- Disable the VBO

This also allows to hide or move around subparts. Before drawing a subpart, you can apply other materials, and call glTranslate/glScale or whatever transformation. I wonder if it's worth the trouble though, since a GUI usually only contains a relative small amount of simple quads.

Rick