Yeah, in the meantime i fought another battle with Reaper (Windows version via Wine), Jack, Wine-VST-Bridge via Cadence and my Toontrack products.
After 3 days of fiddling it worked; i was very happy it worked at all. But stable is definitely not a word that comes to mind.
The problem seems (still) to be Jack-audio with my UR-22. The connection breaks sometimes for no apparent reason and this disconnects the wine-bridge from Reaper which then takes up to 2 minutes until i can work again.
The only solution is to push the buffer size to high values which causes high latency.
For now i switched back to Windows 7, which was really fun to update since Microsoft dropped the support. After that, no more headaches.
When i have some time on my hands again i will try your approach Jostein, just hosting the VSTs via Wine. But again, for me the problem seems to start at Jack-Audio. To modify its variables all day and still ending up with 80ms delay for a stable connection is just so incredibly frustrating.