The VF1 window and frameline is of a 4:3 aspect ratio, not 3:2. It was intended for micro-4/3 Olympus cameras.
I bought one for use on my Fujifilm X10, whose native format is also 4:3. It works great, optically, but like the OP's experience wouldn't slide all the way on to the X10 hot shoe. I ended up having to shave off a bit of thickness from the VF1 mount using a Dremel tool. There's some nonstandard thickness to the VF1. The problem is not the Leica.
~Joe