Twindy,
the only way you can really tell if your vestibular nerve has been already compromised is by taking a balance (VNG) test.
I had a relatively large tumor, which obviously grew very slowly over a period of many years, and I had never experienced any balance problems. Even when the neurotologist examined me thoroughly, he said that my balance looked "pretty darn good". But when I took the VNG test, the result came back as 89% weakness on the AN side. In short, my balance was almost totally gone, but this happened so slowly that the other side compensated. I had no balance problems after surgery. They always provide a walker at the hospital for use immediately after surgery - I only used it for a day or two, and mostly because my legs felt weak.
On the other hand, some people with very small tumors experience very intense balance problems - so every individual is unique. If you have a working nerve, then it is more likely to experience problems, at least in the short term.
There is no real way to predict what will happen after surgery, but my neurotologist had explained that the result of the VNG test can be a pretty good predictor (i.e. if your balance is already gone, then most likely you won't experience balance problems afterwards).
Marianna