Ian wrote,on my timestamp of 17/03/2012 4:46 AM:

I used both types of shutter and found that the vertical shutter was less
liable to shake at slow shutter speeds than the horizontal one.
I think (it was a long time ago) that I compared the Canon A1 (horizontal
shutter) and T90 (vertical shutter). Using a 50mm lens, the slowest
shakefree speed with the horizontal shutter was 1/30 or 1/60 and the slowest
shakefree speed with the vertical shutter was 1/8 or 1/15.

Ah well, that's what you get when you compare a camera 10 years younger than
another. It's got nothing to do with the direction of the shutter and all to do
with a more modern design with better dampening. Besides, the A1 was notoriously
under-dampened in the mirror box and that had a major impact. The only way to
accurately compare those two for shutter vibration is to use each with the
mirror up.

Try using a Konica Hexar or ZI RF with vertical shutter and a Leica with
horizontal and then you'll clearly see the difference.