I thought about this as well and came to the following conclusion.
I believe "B" can't be used with F/2 due to coupling mechanism implementation. The philosophy of the Vitomatic is "look - and turn". Light meter is coupled to the settings and indicator is visible in the viewfinder. You don't have to check the settings to shoot with correct exposure.
The aperture setting ring is actually connected to the shutter speed ring. When you hit F/2, turning the aperture ring turns the shutter speed ring. And the maximum controlled setting is 1s F/2. Here the aperture ring hits the limiter. One can still change the shutter speed ring to "B", but since "B" isn't a defined speed, it can't be used to guarantee correct exposure. And the coupling causes aperture to change to F/2.8 since the ring is, as mentioned, blocked at 1s F/2.
This is a small sacrifice caused by coupling implementation that works on all other settings.
And here are the photos of my Vitomatic IIa. I'm really glad I fixed it.