What would the average temperature be if there were no greenhouse effect from water vapor?
As was pointed out in the question comments, there is no way to know what Earth's average temperature would do without water vapor in the atmosphere.  However, we can estimate the effects of the water, and remove them.  What follows is a rough approximation.

If there were no greenhouse effect, the average temperature of Earth would be about 0F (-18C) instead of 59F (15C). (https://www.giss.nasa.gov/research/briefs/ma_01/)

Water vapor roughly doubles the warming caused by carbon dioxide. (https://www.nasa.gov/topics/earth/features/vapor_warming.html)

Therefore I estimate that without water vapor, the average temperature would be about 30F (-1C).