I think your major problem is taking things a little too far. That vertical slide is a mighty big lump to have on such a small bed, but should still be able to be used, after a fashion.
Many years ago, when money was really tight and small machinery, like hobby mills, just weren't available, a vertical slide was the only real way of getting some semblance of milling done in a home workshop, and during it's time, people managed to perfect it's use and some marvellous creations were produced using them.
But like everything else in the machining world, you have to get to learn to use it correctly. Whatever is relevent to a milling machine must be used with this attachment. Rigidity of the workpiece and a full understanding of the complexities of milling cutters and their uses. Just sticking a cutter into a collet chuck and hacking away at a lump of metal held in the vertical slide will be doomed to failure. Climb milling is usually the first stumbling block, followed by attempting to take too heavy a cut or too fast a feed. Climb milling is easily solved, DON'T DO IT, as to the others, that is a matter of yourself learning how far you can push the limits of the attachments before you start to get unsatisfactory results. With a milling machine, you might be able to take a 50 thou cut, but using a vertical slide, you might find you can only take 2 thou, it all depends on rigidity and setup.
So play about with it a little longer and find both it's and your limitations, and maybe you will find a way for yourself to get good results, but on no account will it ever be as easy or as fast as using a true milling machine.
Bogs