Supposedly saw one guy prior to taking it up myself. According to him I asked what it was and he let me try it, but I had no recollection of this just a few weeks later. In any case I saw him vaping after I took it up. Other than that I have seen a guy smoking an imitation cigarette in a bar where I live.
Generally I do not ask before using it in cafes/bars etc unless they are small, very personal or I am right by the work area. Whenever I have asked the staff have appeared to be familiar with the concept already, and very relaxed about it, which is a good sign.
Of course, I seldom leave the house and tend to be in my own little world when I do, and only remember my glasses about once in two years, so vaping is unlikely to attract my notice unless it happened under my nose.