Can a single impact event trigger a runaway greenhouse?
What your calculations don't account for is the dynamics of the meterological system.  An atmosphere with $10^{18}$kg of water vapour would be a powerful greenhouse, but at the temperatures on Earth, there is no way that the atmosphere could hold so much water vapour, it would rapidly condense.
Instead, in the case of a major impact into the oceans, a lot of water vapour is produced, which rapidly condenses to clouds because the Earth isn't hot yet.  The clouds will tend to cool the Earth (by raising albedo).
The climatic effects will be significant, but you would expect all the water vapour generated to condense fairly rapidly, and far faster than the greenhouse effect of the water vapour.
The Earth has certainly experienced such impacts into the oceans, (this is a Chicixlub level event, expect once every 100 million years or so, and ⅔ will fall into the ocean) and yet no runaway greenhouse has occurred.