e-cigarettes do NOT give off smoke, they give off a water vapour and very little of it at that.
And should smoke detectors be susceptible to water vapour (steam) then don't exhale it, simply swallow it ... it's only water after all.
I went on to e-cigs from the real thing and after 6 months of e-cigs I gave up all together, that was nigh on 4 years ago and I haven't been tempted to smoke since.