Regarding the melting glaciers and icebergs, why isn't the extra water vapor in the atmosphere mentioned in discussions of global warming?
Extra water doesn't mean extra water vapor. Even without this EXTRA water, the water vapor levels will increase, in the current warming world. As its the temperature rise that is making the water turn to vapor. 


  As temperature rises, evaporation increases and more water vapour accumulates in the atmosphere. As a greenhouse gas, the water absorbs more heat, further warming the air and causing more evaporation. When CO2 is added to the atmosphere, as a greenhouse gas it has a warming effect. This causes more water to evaporate and warm the air to a higher, stabilized level. So the warming from CO2 has an amplified effect. Water vapor is infact the largest positive feedback in the climate system (Soden 2005). - Skeptical Science


Intensification of the water cycle hence, is a product of rise in temperature, rather than the water content. Of course we need water to evaporate, but the increase in the water content doesn't correlate to increased water vapor. It might play some role locally in small ponds and seasonal rivers which get dry during summers, but on a global scale with most of our evaporation occurring over oceans (86% of global evaporation), its not significant enough. 

So coming to your question. First of all its not like water vapor is never mentioned - 


  As global temperatures rise, the atmosphere then holds more moisture ... Therefore there is more water vapor available to fall as rain, snow. - thinkprogress.org - September 5
  
  Global warming is moistening the atmosphere - The Guardian - August 13


And if you notice, all the recent news about melting glaciers has been in connected to some recent developments and observations in form of published papers at reputed journals. NASA even conducted a press conference explaining their latest research, which led to a series of news articles on the topic. So, i'm quite sure if there are some significant developments in terms of water vapor research, it will make the news as well.