I bought one, after reading this thread, in the hope that the communication between the C1 and Reaper gets sorted out soon! Don't make me regret it Torsten!
I had the stutter issue using VST3 too. Another plugin I have drops a channel when I use the VST3 version, so I think you're right about Reaper VST3 support being a bit flaky at this stage. I generally just avoid using VST3, I have no idea what the benefits are supposed to be!
Edit: Just had a look at the Softube web site, no mention of Reaper on the "Console 1 Known Issues/Work In Progress" page, I guess it's a pretty low priority