Meanwhile I got it to work
The reason was that I previously installed a wrong virus software on my pc by mistake. I had 2.7.3 on my pc and 2.7.5 on my snow, which seems not to be compatible.
After I installed the 2.7.5 sw package on my pc the virus control panel opens without problem in ableton live as well as in VSThost.
In ableton live I noticed that I can open the virus control only if I have not loaded e.g. the tutorial song. If a song is loaded and I want to open the VC I get the message: "No free VirusTI Snow available: You are already running instances of VC allocating all connected VirusTI Snow devices. You can only open Virus Control once for every device".
I just want to open it e.g. for changing patches or modify patches. Maybe I try to open VC at the wrong place (the PlugIn-Device list). But I didn't find another button to open. Trying those things it often appears that the sound will disappear all of a sudden. You see that the song is played, but you don't hear anything. Only a restart of ableton helps.
In VSThost I am able to play with external midi keyboard while selecting/changing presets from virus rom/ram banks as well as from various patch libraries arround the world. Thats great. Although there is a big latency from pressing the key until hearing the sound. But I assume there are some tuning possiblities in my configuration.
Anyway, great to reach that point.