The Vito IIa is a design "improvement" over the Vito II. It uses a top deck similar to the original Vito B. Like the Vito II, it has a coated Color Skopar lens and usually a Prontor shutter.
Some of the Vito II cameras have a Compur shutter, although you're far more likely to encounter one with a Pronto(r).
The Vito III is very similar to the Prominent -- essentially it is a lighter weight folding version of the Prominent. However, these are not bargains and often sell for more than the Prominent. These usually have the excellent Ultron lens. I can't remember if Voigtlander put a Septon onto the Vito III.
I had a Vito CLR and now have a Vitomatic IIb. Both are rangefinder cameras with selenium meters. The CLR is a slightly larger model and very usable. The IIb is smaller -- it's essentialy a second model Vito B (the one with the van albada viewfinder) that has added a meter and rangefinder. As a result, the IIb is much noticeably heavier than the B.
There is a small translucent dial on the top of the right deck that collects light so the meter needle can be seen inside the viewfinder. It's a nice little camera with many tricks and a coated Color Skopar lens.
Both the CLR and the IIb have a front-mounted shutter release, which Ivor Mantanle calls correctly "an acquired taste." In the end, you generally tolerate it.
The IIb has a rewind lever, rather than knob rewind. I don't remember if the CLR, which also has a Color Skopar lens (at least the one I owned had one), has knob rewind.
I have a short writeup of the
Vito III, as well as a couple other Voigtlander cameras on my site. The Prominent is my least favorite.