[cpdiii]

And loss of the vertical stabilizer will initiate an unrecoverable spin? Why?

The only thing that keeps the aircraft stable relative to forward flight is the vertical stabilizer. That is why it is called the vertical stabilizer. When an aircraft "yaws" left or right the vertical stabilizer gives a counter active correction and thus assumes stable flight. Without this vertical stabilizer the aircraft is inherently unstable. It will without doubt spin in as this become the "stable flight form" without the vertical stabilizer. At this point the pilot is helpless. It is going in!

It should be noted that the B2 Stealth bomber has no vertical stabilizer and is horribly unstable in flight. It can only fly because of the flight computer that inputs corrections to the instability in fractions of a second. A human pilot could not do this. The pilot flies the plane but the computer keeps it stable in flight via rapid micro corrections on the wings control surfaces.

[IronJack]

I understand that the absence of a vertical stabilizer (a "tail fin" for the un-aeronautical) would result in yaw instability. I was more wondering why the aircraft would lose altitude drastically because of it.

Captain Al Haynes flew United 232 without any control surfaces, including yaw, when he lost the hydraulics. He used differential thrust to "steer" the DC-10. However, he did not lose the vertical stabilizer entirely.

I'm sure a spinning aircraft has little pitch stability, but I'm trying to understand why it would immediately nose down and plunge to earth.