Due to IC and emergent cerclage I'm on bedrest for 11 weeks already (31 weeks today).muscles have gone. Does inactivity affect vaginal birth?

Hard to say. In my experience, women who stay in shape do better during labor and delivery, but many couch potatoes also do just fine and there are a lot of factors at work here. It is not like you had a good alternative. Reaching 31 weeks is already a huge success and a little bit longer is even better. Best of luck!